"If faced with the same situation, I would tell the first buyer "if" they buy the item I will give them a discount vial a paypal refund, but only if they got in first to buy the item."

Just tell the buyer to purchase, but wait for an invoice before paying - you can adjust the price in the invoice before they finalise.  I do it all the time.   just remember to NEVER use the re-calculate button before you send cause it isn't re-calculate, it's "rest to original price/postage"