My understanding is, (and what I would have done in your situation) the 2nd buyer who bought the item with instant payment is the rightful purchaser of the item and you should proceed with the sale to them.Even though your first buyer had an interest in his (3rd item) ,it appears he did not purchase it buy it now, instead of buying, he elected to negotiate a price.If the lisitng is active, that is what it is, It is active and available for first come,first served.Your first buyer should have bought the item, and then asked you to alter the invoice for cheaper cost etc, after hitting the buy it now button, and not before.If you offer your first buyer this explanation I am sure they would agree, technically they had not purchased it, they only had the intent to purchase.

Over the years I have had similar requests, some buyers follow through with their intention to purchase, and others do not (they get me to organise multiple items in the one box, then disappear, without ever communicating again.Who ever gives you the money 1st, gets the item.

Other ebayers may see this situation differently,maybe wait for another few replys to come through, then you can make a better informed decision on what you want to do.Remember the first buyer (3 items man) can't leave feedback becuase they did not purchase it, however, 2nd buyer (who paid the $$) can leave feedback, be it negative or positive.

Regards.