Buying Chametz after Pesach

If somebody accidentally bought KTS after Pes from a store, a Jewish owned store that hadn't sold their KTS, what should they do? Naturally, a person would want to return it to the store, is that permitted? Are you allowed to return the KTS that you bought or is that considered also that you're deriving benefit from the KTS by returning it to the store? So, there's really two issues to be discussed here in terms of deriving benefit. So, post can say that that's not called deriving benefit because the person sold me something which they had no right to sell. It had no value. Even though they're not going to return me my money unless I give them back the item, host can write that that is permitted. The mug of Ram though brings up a different issue which is that by returning the item to the store, it might create a situation where another Jew might come in and buy it. Maybe that should not be allowed. Other postkim disagree and they say that it's not a concern. The Kums was there from before. I'm just returning it back to the situation that it was in before. And there are those who would say it's better to just throw it away and avoid all the questions. And the others that say no, it's perfectly acceptable to return it to the store. There usually other doubts here as well. Was the that I'm returning was it owned by the Jew over Pes? Sometimes there's there's a doubt over here. Maybe it was it was received only after Pesak in which case it wasn't included in the penalty at all. So therefore, for many reasons, you would be allowed to return it to the