Gemara

(Mishnah): If Reuven slaughtered a cow (and sold it to Shimon), then it became known that it is a Terefah, what was eaten was eaten. Shimon returns the meat that was not eaten, and Reuven returns the money;

If Shimon sold it to Nochrim or fed it to dogs, (Reuven refunds the money, but deducts because) Shimon must return the value of a Terefah (in place of the meat).

(Beraisa): In the following cases, if Reuven sold food and it later became known that it was forbidden, what was eaten was eaten, and he returns the money:

He sold meat, and it was found to be a Bechor (that was not permitted by a Chacham);

He sold produce, and it was found to be Tevel;

He sold wine, and it was found to be Yayin Nesech.

R. Shimon ben Elazar says, if he sold things that people detest, he returns the money;

If people do not detest them, he (returns the money, but) deducts their value.

People detest Nevelos, Terefos, Shekatzim and Remashim (vermin). They do not detest Bechor, Tevel or Yayin Nesech.

Rishonim

Rambam (Hilchos Mechirah 16:12): If one slaughtered and sold a cow, and after it became known that it was Tereifah, whatever the buyer ate, he ate, and the seller returns the money. What was not eaten is returned, and the seller returns the money.

Rambam (13): If the buyer sold it to Nochrim or fed it to dogs, we consider its price like that of a Tereifah animal, and the seller returns the excess that was paid. The same applies to all similar cases.

Rambam (14): The same applies to anything Asur to eat mid'Oraisa, whether it is forbidden by a Lav or Kares. If it was Asur mid'Rabanan, if the food is intact, he returns it and takes back his money. If the buyer ate it, the seller does not return anything.

Magid Mishneh and Rivash (499, cited in Kesef Mishneh): I did not find an explicit source for this. However, the Gemara listed only Isurei Torah, or Isurei Hana'ah mid'Rabanan. This implies that if was Asur only to eat, mid'Rabanan, if it was eaten, the seller would not return anything.

Magid Mishneh: The Yerushalmi permits doing business with Isurei mid'Rabanan. Perhaps the same applies here. Or, the Rambam has another source.

Rivash: Even though the Rambam has no solid proof, no one after him disagreed.

Poskim

Shulchan Aruch (YD 119:13): If one sold forbidden food, if the buyer found out before eating it, he returns it, and the seller returns the money. If he found out after eating it, what he ate, he ate, and the seller returns the money. If the buyer sold it to Nochrim or fed it to dogs, he pays the seller the price of a Tereifah animal. If the Isur to eat it was mid'Rabanan, or (Taz (13) - this should say 'then, if') the food is intact, he returns it and the seller returns the money. If the buyer ate it, the seller does not return anything.

Shulchan Aruch (CM 234:2): If one slaughtered and sold a cow, and after it became known that it was Tereifah, whatever the buyer ate, he ate, and the seller returns all the money. What was not eaten is returned, and the seller returns all the money.

Shulchan Aruch (3): If there is an Isur mid'Rabanan to eat something, and one bought it, if it is intact, he returns it and gets back his money. If he ate it, the seller need not return anything to him.

SMA (4): Chachamim fined the seller only for an Isur Torah. Also, then what he ate is not considered Hana'ah, for he is pained that he transgressed a mid'Oraisa law. This does not apply to an Isur mid'Rabanan,

Rebuttal (Shach YD 119:27): These reasons do not suffice to distinguish between mid'Oraisa and mid'Rabanan. Also, even if we do not fine for an Isur mid'Rabanan, the seller should refund the difference between the price and the value. This is letter of the law. It is not a fine! Rather, Chachamim did not fine to make someone pay. Rather, they leave the money where it is, like Torah law. The Magid Mishneh connotes like this. Perhaps he means that whenever Chachamim did not enact regarding business, they left Torah law in effect regarding a sale.

Nesivos ha'Mishpat (Bi'urim 3): Since the sale is Batel, the buyer is like one who damaged or ate another's property. Why does he pay the full price, even if this is more than the actual value? Perhaps even though one who ate an Isur Torah needs Kaparah and Teshuvah to protect from punishments, one who ate an Isur mid'Rabanan needs no Kaparah. It is as if he did not sin. If it seems that an Isur mid'Rabanan is being done, we do not ask until after the event (Eruvin 67b). If there is punishment for Shogeg of mid'Rabanan, why do we allow him to sin?! Rather, there is no punishment, and it is as if he ate Kosher food. Since he benefited from Kosher food (mid'Oraisa), he pays the full price. This is like one who benefits, when another loses through this. He pays for his full benefit. If he ate an Isur Torah, the punishment for Shogeg is greater than his benefit.

Chasam Sofer (CM 180): It seems that there is a proof for the Shach. The Rambam holds that Lo Sasur forbids all Isurei mid'Rabanan mid'Oraisa, just Chachamim enacted leniencies about their laws, e.g. to be lenient about a Safek and not to be lashed for them. Eating an Isur Torah is as bad as eating an Isur Torah, just they enacted not to make someone lose money due to them. However, how does he learn from the Heter to sell Isurei mid'Rabanan that Chachamim did not enact to make people lose? Mid'Rabanan laws and stringencies cause many losses! Also, if the seller would not receive the price of a Kosher animal, he did not gain, but he did not lose! If one married a woman without knowing that she is forbidden by a Lav, she gets Tosefes Kesuvah. The Ran explains that she did not need to tell him, for perhaps he would want to marry her anyway. Why is this different than making one stumble in forbidden food? The Beis Meir (EH 116:1 DH v'Ulam) answered that a buyer can ask only the seller, but presumably others knew that she was forbidden, and her husband could have investigated. Since he did not, she can say that she assumed that he wanted anyway. This supports the SMA, who says that it depends on whether one despises the Isur. He did not investigate because he does not despise Bi'as Chayvei Lavin. I can answer my question against the Magid Mishneh. Chachamim forbade business with Isurim lest one eat them. Since they did not decree about Isurim mid'Rabanan, this shows that if one eats it b'Shogeg, this is not like eating an Isur (Torah) b'Shogeg. Therefore, the buyer must pay at least the price of a Tereifah. He pays the full price, because he saved money that he would have paid for Kosher food. To resolve the difference between this and Tosefes Kesuvah of Chayvei Lavin, the Rambam concluded that it depends on what people detest.

Pischei Teshuvah (CM 234:1): The Shach connotes that even if he bought on credit, he must pay as if it was Heter. Sha'ar Mishpat (1) explicitly says so, for the buyer was obligated from the time he bought. He equates this to one who wrote to a Kohen 'I owe to you five Sela'im for Pidyon ha'Ben.' Mid'Rabanan his son is not redeemed, but he must pay, even though he obligated himself through a mistake, since mid'Oraisa his son is redeemed. The Chasam Sofer connotes that if the seller was not yet paid, the buyer need not pay the price of Kosher food. The Rambam and Shulchan Aruch connote like this. The Pri Chodosh holds that even if the seller already was paid, he must return the difference between Kosher and Tereifah. We say that he does not return anything when there is no difference, e.g. Nochri cheese. Therefore, the buyer can surely claim 'I hold like the Chasam Sofer and Pri Chodosh.'

Ohr Some'ach (Hilchos Gerushin 1:1): Perhaps Shogeg about a mid'Rabanan is not an Aveirah at all. Isurim Mid'Rabanan are not intrinsically forbidden. One must guard them due to Lo Sasur. If one was Shogeg, his transgression was not due to disobeying Chachamim, rather, because he did not know, and this is not an Aveirah at all. If one bought Isur, and found out that it was Asur only after he ate it, it is a Mekach Ta'os (mistaken sale) and he gets back his money. He does not deduct for what he ate. The Rambam connotes like the Shach, that if it was Asur mid'Rabanan, he deducts the full value. This is because eating an Isur mid'Rabanan is not considered an Isur at all, like the Nesivos ha'Mishpat explained. However, if so, one may give an Isur mid'Rabanan to another if the other does not know! Surely this is not so! Rav said 'Shmuel would not stumble by giving to me something that I forbid (even mid'Rabanan)' (Chulin 111b). The Rambam (Hilchos Isurei Bi'ah 1:12) says that if one had Bi'ah with Sheniyos k'Mis'asek, he is liable, i.e. to Shamayim (Kesef Mishneh), and he needs a Kaparah. Do not say that Mis'asek with Arayos is different because he benefited, for the Rambam said similarly about eating.

Note: Mis'asek with Chelev and Arayos is liable because he benefited! (Krisos 19a). The Radvaz and Kesef Mishneh (Hilchos Ma'achalos Asuros 14:12) say that the Rambam applies this to all forbidden food. It seems that the Ohr Some'ach disagrees.