Rabba proves that the concept of shinuy being ko'neh for a ganev allowing him to keep the item and return only the value is d'oraysa, but he has a safeik about yi'ush whether it is ko'neh m'doraysa or only d'rabonon. The basis for his question is that we don't find the concept of yi'ush in the torah in the context of theft, only in the context of aveida. Rashi explains that in regard to an aveida, a lost object, the gemara in baba metzia 22b learns out from a pasuk that if it is "lost from everyone", which leads to the yi'ush of the owner, there is no mitzvah to return it. The question of the gemara is whether we learn geneiva from aveida. Now, there seems to be 2 distinctions between the yi'ush by aveida and the yi'ush by geneiva: 1. Yi'ush by aveida is before the finder picks it up, whereas yi'ush by geneiva is after the thief already has it in his hand. 2. Yi'suh by aveida allows the finder to keep it without having to return anything, but yi'ush by geneiva would only allow the thief to keep the object but he would still have to return the value.
On the side of the question that yi'ush is NOT d'oraysa by geneiva, the rationale is that it comes to his hand b'issur, meaning that the thief has it prior to yi'ush which doesn't work even by aveida - so what is the rationale that we do learn yi'ush by geneiva from aveida?
Rashi says that if yi'ush works by aveida prior to the finder picking it up to allow the finder to keep it entirely, it should work by geneiva even though it is already in the hands of the thief at least to allow him to only return the value and not the object itself. Tosafos concurs with this, and explains that yi'ush would in fact work by aveida even after the finder picks it up to allow him to keep the object itself and only return it's value, just as we are suggesting by geneiva. From Rashi and Tosafos it seems that the gemara takes for granted that yi'ush works by aveida even after he picks it up to allow him to keep the object and only return money, the only question is whether we apply this to geneiva as well since it "came to his hand b'issur", meaning that he stole it rather than just found it. But the Rashba seems to understand that the fundamental safeik of the gemara is whether by aveida itself, the yi'ush helps after it has already come into his hands to allow him to keep the object and only return the value. The Ketzos HaChoshen (361:2) explains that the safeik of the gemara is based on the understanding why yi'ush doesn't work by aveida after the finder already picks it up. Is it because the finder becomes a shomer on it by picking it up, which would not apply by geneiva and therefore work even after he already has it in his hand, or is it because anytime the object comes to his hand "b'issur" - meaning before yi'ush, he has no ability to acquire it through yi'ush both by aveida and by geneiva.
What is the rationale behind the idea of rashi and tosafos that yi'ush works before it comes into the persons hands to allow him to keep it without returning anything, yet yi'ush after it comes into his hands only works for the object but not for the value (unlike hefker as tosafos points out which would work completely even if the object came into his hand b'issur)? When one picks up an aveida or steals an object prior to yi'ush, that act itself is an act of stealing since the object still has an owner who expects to get it back. The act of picking it up is mechayev the person to pay back the value of the object. Therefore, Hefker which is a form of relinquishing ownership would relinquish all claims that the owner has on the finder or thief and allow him to keep the object without returning anything, but yi'ush which is only a realization that he will not be getting it back will not exempt the finder or thief from his obligations of reimbursement that he accepted upon picking it up, just allows the finder to not return the actual object. However, if yi'ush occurs before picking up the object, since the object doesn't have an "owner" associated with it, there are no obligations at all on the finder by picking up the object so he doesn't have to return anything.