Baba Metzia 26b - Hashavas Aveida

Rava says that if one takes an aveida prior to yi'ush with the intent of returning it, and then decides after yi'ush to keep if for himself, he is in violation of "השב תשיבם". Rashi understands that he is only in violation of השב תשיבם, because the issur of gezeila only applies at the time that one grabs it (as rashi writes in many places), and the issur of לא תוכל להתעלם applies only at the time that one ignores the object by not picking it up. Rashi understands that the issur of לא תוכל להתעלם only applies until the object is picked up, but once the object is picked up for the purpose of returning it, the only prohibition left to be in violation of is hasheiv tishiveim. The Ba'al Hameor disagrees and holds that just as Hasheiv Tishiveim applies until the object is returned to the owner, so too לא תוכל להתעלם applies until the object is returned. Tosafos cites a braisa in kiddushin 34a that supports Rashi, which compels Tosafos to be go'reis in the gemara clearly like rashi - אינו עובר אלא משום השב תשיבם. The braisa in kiddushin lists the aseh of "השב תשיבם" as a mitzvah that is not time bound, thereby obligating women. The question is that would would anyway be obligated due to the negative prohibition of לא תוכל להתעלם? Tosafos therefore holds that there must be a case where the mitzvah of "hasheiv tishiveim" applies and the issur of לא תוכל להתעלם  doesn't apply, such as our case where the object is taken for the purpose of returning and then the finder fails to return it.