If I understood correctly, you did not sign the debit slip for the incorrect amount.

While UA may technically have no liability for what happened, the truth is that their error caused hardship to one of their 1K customers. It is a constant source of irritation to me that a debit transaction processes almost real time, and that refunds/credits seem to take an entirely disproportionate time to post. You have nothing to lose by requesting they they cover the charge that your bank issued for the overdraft. United seems to have no problem doling out vouchers for most any travel complaint, so I would hope they would at least take that approach to avoid leaving you completely out of pocket.

You might also call your bank. I haven't found banks in the US to be exactly forgiving, but again you don't have anything to lose by explaining what happened and asking them to waive the overdraft fee this once. If they see the credit being processed back in, they may be willing to give you a break.

Good luck!