Negotiating with buyers as a wholesaler

[Sam S.]

A cash buyer just made a verbal offer which I verbally accepted and we shook on, then I sent him the contract and he saw how much I was making.  Now he says "I just ran the numbers again" and now his price is of course lower and he "doesn't remember" making a firm offer at the price we agreed on.  Other than double closing, what can I do to make sure this does not happen?  Have the buyer submit their offer in writing and swear they have completed their calculations and investigations on an "offer sheet" which I will then turn into my contract?