You cant do that. You are obligated to let the auction end and run out of time..
It is a scam when you work with people asking for things to be sold to them outside of ebay.
They get you to agree then you have them send you the money and send the item then once they see it in transit or received the item, they file for their money back.
And if they do that outside of ebay you have no chance to fight for your money or keep it. You take yourself out of the ebay sellers protection program.
If you still have the emails to you that they sent you need to CALL EBAY and get someone on the phone and personally have them look into your messages and see what that guy was doing so they can suspend his account because hes trying to get your item and he will get his money back.
And you run the risk of if your auction remains to the end other people who bid your item up high in price wont get that item you have sold out from under them. Plus thats against ebay rules too.
Now he can go bid on your auction and when he has bid on the auction you can go into the options beside that auction and END the auction early making him the winner of the auction sooner. Then hed pay you,,,, youd send the item and make some money and then be suspicious of him trying to file a case for a refund.
I'd advise you to take his ID NAME and BLOCK him.
Otherwise you just set yourself to be stolen from, break ebay rules and risk getting your own account suspended, and loose the money when he gets his refund.