Gemara

95a (Mishnah - R. Meir): If Reuven gave wool to Shimon to dye red, and he dyed it black, or vice-versa, Shimon pays him the value of the wool that he received.

Inference: He pays only the value of the wool, not the added value of dyed wool.

If he held that Ein Shinuy Koneh (one does not a through a change), he should pay for dyed wool! Rather, he must hold that Shinuy Koneh.

99b (Shmuel): A slaughterer who slaughtered improperly is liable. He is a damager, he is negligent. It is as if he slaughtered not where he was told.

Shmuel (to a Chacham who questioned this): You should have understood that my law is like R. Meir. Why do you ask from a Beraisa that is like Chachamim?!

101a (Beraisa - R. Meir): If Reuven gave wood to a carpenter to make a chair, and he made a bench, or vice-versa, he pays the value of the wood he received;

R. Yehudah says, he pays the lesser of the Shevach (increased value) and the expenses.

R. Meir admits that if he was told to make a nice chair and made an unsightly one, Reuven pays the lesser of the Shevach and the expenses.

102a (Rav Huna): The Halachah follows R. Yehudah.

Rishonim

The Rif brings the Mishnah and Beraisa.

Nimukei Yosef: Rashi explains that he gets only his expenses, but not his full wage. The Ro'oh explains that he is paid like a worker would want to receive to be idle (and not work). If the contracted wage is less than the expense, Reuven pays only it. R. Yehudah fines Shimon for deviating.

Rambam (Hilchos Sechirus 10:4): If Reuven gave wool to a dyer (Shimon) to dye red, and he dyed it black, or vice-versa, Reuven pays the lesser of the Shevach and the expenses. If Reuven does not want the result and requests the value of the wool, we do not heed him. Similarly, if Shimon offers to pay this, we do not heed him, for Ein Uman Koneh bi'Shvach Kli (a craftsman does not acquire improvements that he makes to a Kli).

Rebuttal (Ra'avad): There is no reason to say so.

Magid Mishneh: The Rambam is proper! The Gemara said that Reuven pays the lesser of the Shevach and the expenses! If the dyer wants this, Reuven cannot refuse! Perhaps the Ra'avad means that there was no need to say it. Rashi explains that Reuven can give the wool to Shimon, i.e. for its value. This requires investigation.

Migdal Oz: The Yerushalmi says that if one built on another's property without permission, he cannot opt to take back his rocks and wood. R. Yehoshua ben Levi called this 'an enactment to promote settlement of Eretz Yisrael', but the Stam Gemara connotes that it is letter of the law. The same applies to all similar cases. Also, one can decline an enactment of Chachamim only in certain cases (an enactment made for his benefit). Also, the Halachah follows R. Yehudah. If Reuven may demand back the value of his wool, this is like R. Meir!

Rosh (10:17): Seemingly, the Mishnah means that if the wool was worth 10 and the ingredients were worth 10, if the result is worth more than 20, the Shevach is greater than the expenses, and Reuven pays only 10. If the result is worth 18, Reuven pays only eight. The Yerushalmi explains differently. Shimon receives nothing until Reuven gets (the initial value of) his wool and the Shevach ha'Ra'uy (what it would have increased) had Shimon not deviated. If the result is worth more than the wool and the Shevach ha'Ra'uy, then Shimon receives. The wool and the Shevach ha'Ra'uy are considered principal. This is unlike one who planted another's field without permission, who receives from all the Shevach. The craftsman was given to dye, so it is as if it already increased in value all the Shevach ha'Ra'uy, so one who deviated must pay everything. The Halachah follows R. Yehudah.

Nimukei Yosef: The Ro'oh said that the Yerushalmi requires investigation.

Hagahos Ashri: R. Elchanan explains that if the owner of the wool vowed to give to the dyer 10, and the dyer must pay his workers five, the five is like principal. Even if he deviated, he takes five from the Shevach of the colored wool over the original wall and ingredients. The 10 is principal of the owner of the wool and ingredients. He gets two thirds of any additional Shevach. Even if the dyer paid more than five for labor, due to his mistake, only five is principal. Some explain that first the dyer takes the five that he paid workers, then (if more Shevach remains) the owner takes the five he should have profited, and then the dyer gets the rest of his normal wage, up to five. If more remains, perhaps the owner gets all of it. The same applies to one who planted another's field without permission if it is not meant to be planted. If it is meant to be planted, he receives a half, third or quarter, like sharecroppers in the city. If a Shali'ach was given money to buy wheat where it is cheap, and he bought where it is expensive, the Shali'ach loses.

Poskim

Shulchan Aruch (CM 306:3): If Reuven gave wool to a dyer (Shimon) to dye red, and he dyed it black, or vice-versa, Reuven pays the Shevach or the expenses, whichever is smaller.

SMA (9): This is like the Rambam, who explains like the Rosh originally suggested. Expenses and Shevach are like we say about one who planed another's field without permission. If not, the Shulchan Aruch would have explained. The Yerushalmi holds that Shevach ha'Ra'uy is what the wool would have increased above the expenses and wages of the dyer. The Rema rules like the Yerushalmi, for he holds that it is primary. However, he should have said 'some say that...', for they differ about whether the dyer collects before the owner gets Shevach ha'Ra'uy! This requires investigation.

Beis Yosef (DH v'Kivan): If the Bavli held like the Yerushalmi, it would have specified. Also, the Yerushalmi is not so clear. R. Elchanan explained it unlike Tosfos and the Rosh. Therefore, the Rambam's Perush is primary.

Rebuttal (Darchei Moshe 2): Tosfos and the Rosh rejected Rashi due to the Yerushalmi, so we cannot say that the Bavli argues. We follow the Tur, who rules like the Yerushalmi.

Shach (5): The Rema did not say 'some say that...' because the intent of the Rambam and Shulchan Aruch is not so clear. The Beis Yosef said that R. Elchanan explained the Yerushalmi unlike Tosfos and the Rosh, i.e. and according to him it is not unlike Rashi. Rashi derived from the Mishnah that the dyer gets at most his expenses, but not his full wage.

Rema: The Shevach ha'Ra'uy had he not deviated is included in the principal. We calculate Shevach and expenses without this.

Shulchan Aruch (ibid.): If Reuven does not want the enactment and asks to get the value of the wool, we do not heed him. Similarly, if Shimon offers to pay this, we do not heed him, for Ein Uman Koneh bi'Shvach Kli.

SMA (10): The enactment is for the owner and the dyer. Neither can retract.

Shach (6): The Maharshal says that this is when the wool did not increase in value at all. If it increased, surely the owner may demand its original value! This is wrong. Perhaps the owner does not want to toil to sell it! The Maharshal says that the Ra'avad's opinion is primary; the owner may demand its original value. I disagree. R. Yehudah's reason is that the dyer did not acquire it! The dyer has the lower hand only because he seeks to take from the owner, and he deviated. Surely the owner cannot take from the dyer!

Gra (6/7?): Shmuel said that his law is like R. Meir. If the dyer were liable, Shmuel's law would be even like R. Yehudah!