Commentary for Bava Kamma 15:20

If you were silent, etc., but if not, I will return the bill [of sale] of the inferior quality [land] to its [previous] owner. The circumstances we are presently dealing with in the Gemara are where the purchaser bought the finest quality of the debtor/seller last. According to the general rule governing the collection from a debtor who sold his property, all claimants can collect from the very last field that was sold by the seller/debtor. The Braita is telling us that the general rule can be overridden by the purchaser who can threaten to return the document of the poorest quality back to the debtor/seller. If he should do that they would all be forced to collect from the poorest. He agrees not to take advantage of that option if they will each be satisfied with their original right. The victim of damages will collect from the finest, the creditor from the medium quality and the k’suboh from the poorest.
This is a bit of an over-simplification. Actually the three claimants are not one group of collectors that must submit to collective bargaining. Each one has his own claim and the debtor has to deal with each one of them individually. Tosafot will now explain how each of the claimants is dealt with.
Since this is merely a threat why does the k’suboh have to surrender her right to collect the finest because of the threat? According to the general rule that one collects from the last property sold by the debtor she has been upgraded from the poorest to the finest. Why should she submit to accepting the poorest in face of the purchaser’s threat? She should say “when you give back the document”; I will accept the poorest from the seller/debtor. There may be a reason that the purchaser really does not want to return the document to the seller/debtor. The woman collecting her k’suboh should have the right to force him to make that move. After all, he may back down.
Even though a woman collecting her k’suboh would in any case receive the poorest, and the purchaser is not offering her any upgrade to entice her to accept his offer, even so, she cannot say “when you will return the document I will accept the poorest” in order to force him to improve her position, because he is not curtailing her rights. Ultimately, the Rabonon said that she could collect her k’suboh from the poorest and he is not offering her anything less than that. As long as he is not infringing on her original right, she must accept his offer. It is precisely for this reason that the purchaser cannot force the creditor to accept a downgrade.
However, as far as a creditor is concerned it cannot be said that he must accept the poorest instead of the medium-quality because of the claim of “if you will be silent etc.”, since the purchaser is curtailing his rights to collect from the medium-quality, the creditor can say to the purchaser “when you will return the document to the seller/debtor I will accept the poorest from him”. For now you are also only offering to give me from the poorest which is a downgrade. Why should I settle? Let us see if you will really return the document to the seller/debtor.
We see that when the purchaser attempts to downgrade the position of the claimant without offering anything in return, the claimant can argue: when you will return the document to the debtor/seller I will accept the poorest from him. The Gemara will soon ask that the purchaser should be able to downgrade the victim of damages with the threat of “if you will be silent and accept medium quality, fine. If not I will return the document of the poorest to the debtor/seller and you will be forced to collect from the poorest. The Gemara does not ask that the purchaser should attempt to downgrade the creditor as well with the same threat.
It is for this reason, that the purchaser cannot downgrade the creditor from his right to collect from the medium-quality without offering him something in return, that the Gemara soon asks that the purchaser should be able to downgrade the victim of damages, only from the case of damages where the purchaser is willing to at least give him medium-quality, so the victim has some reason to compromise and accept the medium instead of being forced to accept the poorest. The Gemara does not ask about the creditor as Rashi explained that he should be compelled to accept the poorest. Since he is not being offered any more than the poorest, he has no reason to compromise and accept the offer and he can say “when you return the document of the poorest quality to the seller/debtor, I will accept the poorest from him”.