That's Odd!
There were problems with the system a couple of days ago. Is that when it happened?
There are several ways to fix it.
One is to cancel the transaction and probably the best if you want to recover all your eBay fees.
Another is to ask the buyer to pay and then issue a refund for the overpayment.............. and probably the best if your think your buyer will go MIA.
A third would be to call eBay and ask that they repair the mistake............ which they might not be able to do and so it's probably a waste of your time ...... :).