it might sound harsh, but making the rules to protect people that are unable to understand what they are doing is not the right way... let them drop out and start again, if you're talking about brand new players, they don't lose much, if a guy like this won't try again he'd drop out either way sooner than later...
In that case you dont need transfer percentage cut from earlier sell. While this thing offers me a freedom in correcting my bad purchase. Remember that there are lot of skills and lot of concepts which player can be contribution for the team, so is great that I can sell immediately someone who dont fit into schedule.
if you buy a player with low salary then those 4 weeks will have almost no impact, this is the case for begginers... players with salaries around 40k are bought mostly by experienced managers so I expect them to be responsible for their actions... so the suggestion in fact hits only mutant players, which is exactly the idea
no changes are needed in the transfer fee rules, those solutions might work together