I think I've encountered this type of problem before with a forgotten exchange, I can't remember its name but I understand that problem really exists. One manual solution is this:
For instance, if they have $100 in ETH and want to withdraw $70, you can recommend them to withdraw $80 and empty their balance instead of leaving a temporarily non-spendable amount of $10.
You have to recommend the users to withdraw as much as possible all that they have and put a reminder that remaining funds might be at loss so they'll be forced to withdraw all of it. Well, this is for the manual part and reminder but technically, it goes to the devs part.