It is all about the money. United is willing to sell him, Betis is willing to buy him, and Antony is willing to move. The only thing left is the small detail of how much and of course Betis is not a wealthy club, so it is not an easy thing to buy him out right, but they can try.

Maybe make it a loan with buy obligation at the end? That way, instead of buying the whole money right now, they can make instalments, pay some right now, pay next season and maybe even later, that way they won't have to be forced to pay right away. Not sure if United would accept that, but it would make sense to stay on the table for both teams, and figure this out. Whatever Betis can pay, the maximum, should United accept and move on, and not look for more by another team.