Rif Nedarim 9b {27b}

9b

{Nedarim 27b continues}

ולרב הונא מכדי אסמכתא היא ואסמכתא לא קניא שאני הכא דמיתפסן זכוותיה ולאו אסמכתא היא
והתנן מי שפרע מקצת חובו והשליש את השטר ואמר אם אין אני נותן מכאן ועד שלשים יום תן לו שטרו והגיע זמן ולא נתן
ר' יוסי אומר יתן
ור' יהודה אומר לא יתן
והא הכא דמיתפס שטריה ביד השליש
ואמר רב נחמן אמר רבה בר אבוה אין הלכה כר' יהודה דאמר אסמכתא קניא
שאני הכא דאמר ליבטל זכוותא
והלכתא אסמכתא קניא והוא דלא אניס והוא דקני מיניה בבית דין חשוב

And to Rav Huna let us see: It is an asmachta {since he never thought it would really be in effect} and an asmachta does not have the force of a legal transfer!? It is different here, for his rights are deposited, and so it is not an asmachta.
But they learnt {tnan}: One who pays off part of his debt and places the bond in the hand of a third party, and says "If I do not give from now until 30 days, give him {back} his bond," and the time arrives and he did not give, Rabbi Yossi says: He {=the third party} should give {the bond back to the creditor}. And Rabbi Yehuda says: He should not give.
And yet here is a case where his bond is in the hand of the third party {just as this other one's rights are deposited in Bet Din}. And Rav Nachman cited Rabba bar Avuah that the halacha is not like Rabbi Yehuda {our gemara: Rabbi Yehuda, and it seems likely that our girsa of Rif here must be a mistake}, who said that asmachta is a valid acquisition!?
It is different here, for he said to nullify his rights.
And the halacha is that asmachta is a valid acquisition.
And this is where he was not unavoidably prevented. And this is where he acquired it from him in an authoritative Bet Din.