Manchester United succeeded to sell a star player they no longer needed and released a little bit pressure on salary budget too. With no plan from Ruben Amorim for Antony, keeping him in the club, paying him salary are very unreasonable actions and no club want to do this if they can find an exit gate for the player. Both Antony and Manchester United actually had to step back a little to finish this transfer deal with Real Betis.
Antony isn't suited to playing in the Premier League with Man United. He's better suited to La Liga. This can be seen in his ability when he was loaned to Real Betis, even though his performance at Manchester United was not that good. Keeping him at the club won't help Man United grow. They really need to make different changes with suitable players.
I think this is a wise decision, not only to save Man United money but also to consider the player future, making it easier for Antony to move without too much drama.