The prevailing argument in favor of cash in trades seems to be that it doesn't matter. That's a horribly weak argument since, if it didn't matter, those in favor of it wouldn't care. If my every-last-dollar $20M bid for a top IFA loses out to a $21M bid by someone who was handed an extra $2M in cash, it matters. If I lose a playoff game to a player the opponent could only afford because he was given $5M extra in cash, it matters. The original question was what's wrong with it, to which the answer is it gives some teams a higher budget than others. Most worlds do not have cash-in-trade rules other than limiting it to contract value, so it's easy enough to play in leagues where gifts are allowed. That doesn't mean someone is wrong to oppose such deals, unless there is also a rule banning vetoes or trade discussion in WC.