Before answering your question, it should be stressed that this is not a matter of Ribis d'Oraisa, since it was not fixed, but at worst, Ribis d'Rabanan (Ribis Me'ucheres, as we shall see).

The Rashba (as well as a number of other Rishonim) deals with this issue. He explains that in the Seifa, where the Shechiv-M'ra added 'be'Chovo', he does not receive more than his debt because it is Ribis Me'ucheres. This is not the case in the Reisha he adds, where he merely said 'li'Peloni Ba'al-Chovi'. This does not insinuate that he is giving him the gift on account of the loan (since he did not mention the debt), and 'li'Peloni Ba'al-Chovi' merely indicates that he likes him because of the many favours that he did for him.

Incidentally, there is a similar Gemara later on 145a & b, in connection with the 'Shushbinus' (where Reuven attended Shimon's wedding and gave him a gift, obligating Shimon to reciprocate when he gets married). The Beraisa there rules that, although reciprocation is considered a debt, it is not subject to Ribis, so Shimon is permitted to give Reuven a larger gift than the one that Reuven gave him). And the Gemara explains that this ie 'because he did not give it to him as Ribis (only in the form of a gift - Rabeinu Gershom).

Perhaps we need to relearn 'Eizehu Neshech', but that's what the Rishonim say.

Be'Virchas Kol Tuv,

Eliezer Chrysler