Baba Basra 149a - Rava with the Money of Issur Giyura

The gemara says that Rava was trying to get out of returning the money of Issur the ger, so that when he dies, Rava is able to acquire the money from hefker and therefore didn't want to give it to R' Mari (who was the biological, but not halachic son of Issur).

Some of the daf learners found Rava's actions to be not so typical for a person of his stature. I think that Tosafos here and in kiddushin shed some light on this issue.

The gemara in kiddushin says that when one borrows from a goy who later converts, sometimes returning the money to the biological (but not halachic) son of the ger is considered - רוח חכמים נוחה הימנו, meaning that chazal are happy with his actions, and sometimes it is not considered to be רוח חכמים נוחה הימנו. In a situation where the son was conceived prior to the father's conversion, but his birth was after the fathers conversion then it is recommended to return it, but if the birth was also prior to the conversion then the chachamim are completely indifferent as to whether or not it is returned (rashi). Rashi explains that when the birth was after the conversion of the father, he is easily confused with a halachic child of the father, therefore we return the money to him so as not to make people think that even to a halachic child the money doesn't have to be returned.

Tosafos points out that if we return the money to any child who was born after the father's conversion, Rava should have returned the money to Rav Mari, the biological son of Issur (who was born after his father's conversion)? Rabbeinu Tam answers that the idea of returning the money is to show hakaras ha'tov to the goy for lending you the money and he would surely want the money to be paid back to his son. But, in Rava's case, Issur wasn't doing him a favor because it wasn't a loan, rather it was a pikadon on which Rava was doing a favor for Issur so there was no reason to return the money. The difficulty with the distinction of Tosafos is that it shouldn't matter whether or not the child was born after the father's conversion, because even if he was born before the father would want the money to be paid back to his son? Why does the gemara seem to hold that even by a loan where there is an issue of hakaras ha'tov, it is only recommended to pay it back to a child who was born after the father's conversion?

Alternatively, Tosafos in kiddushin offers a completely different approach than Rashi. A child who is born after the father's conversion is easily confused with being a biological son of the father, which can cause confusion in the realm of yibum. Therefore, the chachamim suggest to avoid returning loans to him so that he will not be confused with a regular halachic son. According to this approach, it is very understandable why Rava didn't return the money. He wanted to make it clear that Rav Mari was not a halachic child of Issur. The difficulty with this peshat is that Rava expressed dismay - קא מגמרי טענתא לאינשי ומפסדי לי, which sounds like he wasn't concerned with confusion in Rav Mari's relationship to his biological father, but rather concerned over the loss of money?