I have a customer that jumped in at the last minute and outbid another buyer now they want to cancel

[wooden_flower]

I would cancel - no good every comes from trying to force a buyer to complete a purchase. 

You can then offer the item to next high bidder, if that buyer wants to see a second chance offer.  After cancelling, see if 'make second chance offer' is an available action for that item in your sold list.

[gosimus]

@birl3 

< now they want to cancel >     

You have an excellent answer from @wooden_flower but I just want to add – maybe it isn't necessary to remind you – but be sure to put that canceller's username in your Blocked Buyer List, at  https://www.ebay.com/bmgt/buyerrequirements