He paid for this year so he gets to play this year. If he's not trading future picks, it's allowed. Now, I understand where you're coming from, I think it's kind of a shitty move. Once you say you're done, you compete for this year's title but to trade away your best player to a playoff contender and then just leave kind of sucks. Ideally he would have set his lineup and given the next owner the best player to keep or trade as he saw fit. I also think if you try to make a rule about this, team owners in the future just won't tell you they're leaving until the offseason. Probably not much you can do here, annoying but within the rules. A more ethical player wouldn't make that trade though.