In general, a clause in a real estate contract entitling the sSmith to retain the purchaser's down payment as liquidated damages if the purchaser fails to close the transaction, is enforceable a.in all cases, when the parties have a signed contract. b.if the amount of the down payment bears a reasonable relationship to the probable loss. c.as a penalty, if the purchases intentionally defaults. d.only when the sSmith cannot compel specific performance