Abaye and Rava argue whether the beis din simply notifies the person who retracts after the payment has been received that he will be in violation of the "mi she'para", or whether they actually curse him. Rambam and Shulchan Aruch (204:4) rule like rava that the beis din actually curses him. Rava explains that although there is a prohibition to curse - ונשיא בעמך לא תאור, that is only when one behaves like an עושה מעשה עמך. But this person who is retracting after paying and is not sticking to his word (and violates - שארית ישראל לא יעשו עולה ולא ידברו כזב - rashi), is not considered one who follows ma'aseh amcha, and it is permitted to curse him. Tosafos points out that one who retracts on a sale is not considered עושה מעשה עמך in the sense that you can curse him for this specific act that he is doing, but we don't regard him as totally excluded from the prohibition to curse him and therefore one cannot curse him for no reason. At first this seems strange because whether he is or isn't an עושה מעשה עמך should be very clear, without gray area. However, it seems that Tosafos understands that sometimes within a particular action a person can be regarded as one who is not עושה מעשה עמך, but in every other are he is a עושה מעשה עמך. Therefore, within the action that he isn't עושה מעשה עמך, one can treat him in that way, but not in other areas.
I would like to suggest a slightly different explanation of tosafos. The purpose of the mi shepara is to discourage him from backing out. One who backs out of the deal can still be talked into maintaining the deal, and the purpose of the mi shepara is to "convince" him to maintain it. Therefore, one who says that he is backing out has not completely lost his status of עושה מעשה עמך, since he may still decide to maintain it. Therefore it is assur to curse him for no reason. Why can we curse him with a mi she'para? Because the curse is only effective if he persists in backing out, in which case he is not עושה מעשה עמך. But, being that he may change his mind and maintain the deal, one cannot curse him for no reason, since if he decides to maintain the deal he will be a עושה מעשה עמך and it is assur to curse him. This approach would only work based on the approach mentioned in the shulchan aruch and rama that we don't use the language of "הוא יפרע ממך" to direct the curse at him specifically, rather we say "הוא יפרע ממי שאינו עומד בדיבורו" (machlokes rambam and rosh - see sm"a) - the curse therefore is only binding on condition that he follows through with his retraction, but if he maintains the deal he is no longer a "מי שאינו עומד בדיבורו".