Not a lawyer, but I would have thought to cancel the sale, the contract would probably have to say something like, 'subject to international clearance within 2 weeks', or similar.
It does not however make us look good with the player, the selling club, or any other players and clubs we would want to do business with.
Despite the huge salaries, I am sure if we want the best players to want to come to us, we have to treat them a bit better than, 'sorry, last-minute impulse buy, did not read small print about how it worked, returning for refund'.