If you're limiting it to the last week of the season then don't make the player available until the start of the next season, but at the same % that you bought him.
So buy a 99% player with 3 days to go on a pre contract, you can't play him yet and at the start of the next season you get to sign the 99% player
Would make planning the new season a lot easier, especially for getting good Q young players.