Buyer wants to use cash out of his mattress

We are selling a property that we will have capital gains on. We have a buyer (an old guy) that has 50k in cash. He wants to give us 500 $100 bills that he says he has from selling his car and have the “on the books” purchase price reduced by 50k. We are pretty sure the old guy is clean but who knows about the guys he sold the car to. This is so bizarre and we are going through all the pros and cons. So potential cons that we have come up with possible counterfeit, and how do we get rid of that much cash. We are leaning away from accepting the cash but if we don’t take the cash the whole deal is likely to fall through. What are we missing?

Edit: forgot to add that the guy is a car collector. He is buying our property (a luxury car condominium) to house his collection of 25 motorcycles. He has three other buildings he owns to house his car collection

Edit #2: he wants the property turn key

Edit 3: we have accepted an offer from a different buyer and have let this buyer know that we are not willing to provide documents that falsify the sale price. Thanks everyone for all the advice!