At the bottom of 8a the Gemara says that the one who bought the property could tell the people who want to collect to either take what they normally would get or else he'll give the zebores back to the original owner in which case they'll all be stuck getting zebores. Why is this a legitimate threat? the original owner sold it because he wanted to sell it and it was in no way a mekach ta'os, who says that he'll agree to take it back? (and even as a present, maybe he'll say sonei matanos yechyeh)
thank you so much for your previous answers, and i'm looking forward to this one as well.
Aaron Kaplan, Ramat Beit Shemesh
Tosfos (8a DH Machran) learns that the Gemara does not mean to suggest that the field would be sold back to the original owner. Rather the Gemara means that the present owner could hide his contract and make out as if the field was never sold.
The Ramban (Milchamos) explains that the Gemara means that the present owner can force those who want to collect their debts from his property to negotiate a settlement by threatening them that he will sell the land back to the original owner. They can of course to call his bluff and tell him to go ahead and give the property back, but they may not wish to take this course of action.
However, the Rosh (1:3) explains that the Gemara means that since the owner of the property can sell the property back he has the rights as if he had already sold it back. Your question is valid according to this understanding of the Gemara.
The Avnei Choshen (119:5) answers that since the original owner owes money to his debtors, he is duty-bound to buy back the property in order to pay of his debt. The Levush Mordechai (4) says that we could say the Gemara does not mean that he will sell the property back to the original owner, rather that he will pay back the debt from his property on behalf of the original owner and we would look at the payment as if it were from the original owner.
Dov Freedman