Mishnah Berurah — משנה ברורה

Immediately - See in Siman 658 Se'if 5 (Orach Chaim 658, 5) regarding a Lulav and in Pri Megadim there. And from there we can also learn regarding our matter (ie. borrowing a טלית) that one needs to return it to the owner in a timely fashion, which is: if the owner fulfilled his obligation [with the טלית] today, he (the borrower) should, at the latest, return it the following day before the prayer service so that the owner will have [again] it to fulfill his obligation. But if he did not return it to him, and the owner does not have another טלית to use, it would seem that the "gift" (ie. this that the owner gave it to the borrower as a gift on condition that it be returned) was nullified retroactively and it not called "your garment" [anymore] [and even though he (the owner) still has a טלית קטן on, nevertheless, it makes sense that he did not have that in mind when he gave his טלית to the borrower], and this is not like what the sefer Netiv Chaim wrote. It's also explained over there (by Lulav), that after he (the borrower) fulfilled his obligation, he must return it to the owner as a gift in order to transfer ownership [back] to the owner, because it became his (the borrower), and it's obvious that the same thing applies here (to טלית). It seems obvious to me that if the טלית or one of the ציצית got ruined when it was by the borrower, even though it was an accident and [when] he returns it, it's not in the same condition as when he borrowed it, he does not fulfill his obligation in the mitzvah (since there was a stipulation that it was a "gift on condition that it be returned", and the borrower here is not fulfilling the stipulation because he's returning something different than what he borrowed). See further in Siman 658 Se'if 4 and in Chiddushei R. Akiva Eiger there....