Hi PSV....Eric here.
I would set another final date, and include that if the buyer does not hold his end up that $xxxx of the deposit will be kept. And the car re listed for sale at that time. The amount is up to you. Just be sure to put the "new" agreement in writing via email. Ask the buyer for a reply or make sure you get a "read" confirmation on the one you send.
Then everything is in writing. And, you have given the buyer options to work with. Also covered yourself too.
Good luck!
