The Ri says that the reason not to return the loan by a case of horasa shelo b'kedusha v'laidaso b'kedusha is that it may lead to a problem regarding valid Yibum. But in order to do have such a problem, there needs to be another son who really did have to do yibum, in which case that other son is a full fledged heir and would legally inherit the loan, thus rendering the Gemara's entire discussion irrelevant!
Tzvi Hertzberg, New York, USA
This is a very strong question!
It seems that one simply has to answer that the Ri is discussing the reason why one may not return the loan from the point of view of the dangers that may arise concerning prohibited marriages. The Ri agrees that according to financial Halachah one should also not return the loan, as you write.
To put this in different words, the Ri is telling us there is "Isura" involved in returning the loan as well as "Mamona" reasons. It may be possible to find Nafka Minas betweeen these two aspects. For instance, if we were not sure whether there was another brother, but merely had a small "Chashash" that another one might exist, from the financial aspects it might be possible to return the loan to the Ger, while concerning Isura we sometimes are worried about more remote possibilites than we are for Mamona.
The truth is that further thought is required on this important question but I will leave it here for the moment.
Kol Tuv,
Dovid Bloom
I heard from a Talmid Chacham a simple answer to your question.
The Ri does not mean that in this particular case the son who is Horaso she'Lo b'Kedushah and Leidaso b'Kedushah has a full-fledged brother from the same father, since in such a case one clearly must return the loan to this second brother. Rather, the intention of the Ri is that if one returns the loan to this not-full-fledged brother, then people will mistakenly think that the Halachah in general is that such a brother is full-fledged, and later, in a different case, they will wrongly say that such a brother may do Yibum with the widow of a full-fledged brother.
This answer (saying that the Ri is making a general statement) fits better with the Gemara, because if one says that the Ri is discussing specifically a case where the half-fledged brother has a full-fledged brother, then one is making an even bigger "Ukimta" than the Gemara made until now. The Beraisa states that somebody borrowed money from a Ger whose sons converted with him, and the Gemara limited this to a son who was Horaso she'Lo b'Kedushah and Leidaso b'Kedushah. If we say that the Ri is referring specifically to a scenario where the son has another brother, this would be confining the Beraisa even further.
Kol Tuv,
Dovid Bloom