The Gemara explains that Abaye's ruling applies only when the heirs did not
specify why they were giving the money to the lender. However, if they
explicitly told him that they are giving him the money in order to *purchase*
back the land, then 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 towards the loan, and he no longer has any lien on
the property).
Rashi and the Rishonim learn from here that Beis Din may only *force* heirs
to pay their father's debt when they inherited land. If they inherited only
money, then although it is a Mitzvah for them to repay (as Abaye states),
Beis Din cannot force them to repay.
On the other hand, the Gemara (86a) says that Beis Din does force one who
owes money to pay his debts. When can Beis Din force a person to pay his
debts, and when is it only a Mitzvah?
(a) TOSFOS (86a, DH Peri'as Ba'al Chov Mitzvah) and the RAMBAM maintain that
if the heirs inherit land, then Beis Din forces them to pay their father's
debts. If they inherit only mobile property (Metaltelin), there is a Mitzvah
to repay the debts, but Beis Din does not force them to repay. If they do not
inherit anything, then there is not even a Mitzvah to pay their father's
debts.
The logic behind this is that Beis Din enforces the lien on the property that
is inherited, since the lender has monetary rights to the land which is
secured as the lien. Since there is no lien on mobile objects, if the heirs
inherit only mobile objects, they are required to pay back the debt only to
keep 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* people to keep this Mitzvah, though, since the reward for this
Mitzvah is explicitly stated in the Torah. (This is *not* the way Rashi here
understands the Mitzvah to repay one's father's debts.) If the children do
not inherit anything from their father, they are not required to pay his
debts since the Halachah does not require one 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 mobile objects,
Beis Din *does* force them to pay their father's debts. When they inherit
nothing from their father, then there is still a *Mitzvah* to pay their
father's debts, but they are not forced to do so.
These Rishonim understand that our Gemara is discussing a case where 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 mobile property? It cannot be that there is a lien even on mobile
property, and therefore the lender has monetary rights to the objects that
the heirs inherit. We know that no liens take effect on mobile property.
Furthermore, the Gemara says explicitly that "Metalteli d'Yasmi Lo Mishtabdi
l'Ba'al Chov" -- the mobile property of heirs are *not* Meshubad towards the
payment of their [father's] lender (Kesuvos 92a)!
In answer to the first question, the KETZOS HA'CHOSHEN (39:1) explains that
the Rashba holds that the principle that there is no lien on mobile property
is only mid'Rabanan; mid'Oraisa, though, all property, whether land or mobile
property is Meshubad to the lender. As for our second question, the Gemara on
92a only means that mobile objects *purchased from a borrower* the lender's
lien not take effect to collect the object from the buyer.