I'd try asking the buyer first. If I were the buyer and hadn't already paid it, I'd pay it. Most people aren't out to cheat the system. If I misunderstood and he'd already paid it at closing, then I agree that he won't pay it and shouldn't have to double pay, of course. But if he hasn't paid it, then I can't imagine that he wouldn't pay. A simple letter to the buyer explaining the situation may elicit a check and problem solved.

Good luck!