Presumably the buyer signed and is operating under an earnest money contract until closing is completed.

These are a bit one-sided, as different penalties are provided for each of you.

You cannot back out of the sale: if you try, he'll sue for specific performance.

Contrast that harsh remedy with what happens if the buyer changes his mind: in that event you cannot force him to go through with the sale, you only get to keep the earnest money payment he tendered at the beginning, which is usually a token amount.

Fortunately the market is sound and if he backs out you should have no trouble finding another buyer.