No-one saves wages by selling players, even if they let them go for free... Not unless the buying club pays the player more.
When a player is a sold, their contract is either cancelled by both parties, which happens in the case of a player getting a better deal at their new club, OR, the difference (or what is agreed to by the player) between the remaining contract they're on, and their contract at their new club is paid off, either in full, or instalments by the selling club.
Man Utd are not ever saving any of these wages. They might be able to account for them differently but that's about it.