1) COLLECTING A DEBT FROM THE HEIRS OF A BORROWER
OPINIONS: The Gemara records a case of a man who died with a debt of 100 Zuz. The man had land worth 50 Zuz. The lender came to the man's heirs to collect his money and took the land worth 50 Zuz. The heirs then paid him 50 Zuz in order to get the land back. The lender collected the remaining 50 Zuz by taking the property which they had just bought back from him. Abaye ruled that the money which the heirs had paid (in order to retrieve the land from the lender) was a fulfillment of their Mitzvah to repay their father's debt, and it was not a purchase of their father's property. That is, by giving money to the lender they were simply asking the lender to settle for cash instead of taking their land as payment of the loan. Consequently, the lien on the land was still binding and the lender was entitled to take it as repayment for the remaining debt of 50 Zuz.
The Gemara explains that Abaye's ruling applies only when the heirs do not specify why they are giving the money to the lender. However, if they explicitly tell him that they are giving him the money in order to purchase back the land, the lender may not expropriate the land again in order to collect the remaining 50 Zuz (since his lien was "used up" through his collection of the property as partial repayment of the loan, and he no longer has any lien on the property).
Rashi and the Rishonim infer from here that Beis Din may force heirs to pay their father's debt only when they inherited land. If they inherited money, Beis Din cannot force them to repay (although there is a Mitzvah for them to repay, as Abaye states).
However, the Gemara earlier (86a) says that Beis Din may force a person who owes money to pay his debts.
When may Beis Din force a person to pay his debts, and when may Beis Din not force a person to pay his debts?
(a) TOSFOS (86a, DH Peri'as Ba'al Chov Mitzvah) and the RAMBAM maintain that when the heirs inherit land, Beis Din may force them to pay their father's debts. When they inherit only mobile property (Metaltelin), they have a Mitzvah to repay the debts but Beis Din does not force them to repay. If they inherit nothing, they do not even have a Mitzvah to repay their father's debts.
The logic behind this is that Beis Din enforces the lien on the inherited property since the lender has monetary rights to the land secured as the lien. Since there is no lien on Metaltelin, if the heirs inherit only Metaltelin they are required to pay back the debt only to fulfill the Mitzvah of Kibud Av. (See Kidushin 31b, where the Gemara says that one is required to honor his parents after their deaths.) Beis Din does not force a person to fulfill this Mitzvah, however, since the reward for this Mitzvah is explicitly stated in the Torah. (This is not the way Rashi here understands the Mitzvah to repay the debts of one's father.) If the children inherit nothing from their father, they are not required to pay his debts since the Halachah does not require a child to spend his own money for the Mitzvah of Kibud Av. (HAGAHOS ASHIRI to the Rosh 9:14)
(b) RABEINU CHANANEL and the RASHBA (86a, see also RAN) disagree with the other Rishonim. They maintain that when the heirs inherit Metaltelin, Beis Din does force them to pay their father's debts. When they inherit nothing from their father, there is still a Mitzvah to pay their father's debts but they are not forced to do so.
These Rishonim understand that the Gemara here discusses a case in which the children did not inherit anything, and therefore Beis Din does not force them to pay their father's debts, but nevertheless it is a Mitzvah for them to do so (in order to show honor to their father, "Kevod Avihem").
Why, though, does Beis Din force them to repay their father's debts when they inherit Metaltelin? It cannot be because there is a lien on Metaltelin which gives the lender the right to collect the objects which the heirs inherit. No lien takes effect on Metaltelin. Moreover, the Gemara says explicitly that "Metalteli d'Yasmi Lo Mishtabdi l'Ba'al Chov" -- the Metaltelin of heirs are not Meshubad towards the payment of their father's creditor (Kesuvos 92a).
To answer the first question, the KETZOS HA'CHOSHEN (39:1) explains that the Rashba maintains that the principle that no lien takes effect on Metaltelin is only mid'Rabanan. Mid'Oraisa, all property -- whether land or Metaltelin -- is Meshubad to the lender.
(As for the second question, the Gemara on 92a means only that when Metaltelin are purchased from a borrower, the lien does not allow the lender to collect the object from the one who purchased them.)