Perhaps I am misunderstanding the question, but it seems the seller misunderstands how real estate closings work (either thru escrow companies or lawyers depending upon your state). Unless you were to hand the seller cash in a parking lot late at night any real estate transactions goes thru an escrow/title company or lawyer. They will receive and hold the bank's money until all parties sign and the deal is recorded. They then give the seller his proceeds, usually via check or bank transfer. If seller asks THEM for cash I'm sure they will say "ya know, we don't keep hundreds of thousands of dollars in our petty cash drawer so we gotta send ya a check". In any event that part of the transaction is not your concern. They won't release funds to the seller until he gives them his social security number and they are required to send info to the IRS that they just gave Mr. Seller a bunch of money, cash or otherwise.
Hope that helps