S sold to B a car for P1M. The condition for the sale is for B to execute a chattel mortgage on the thing sold. The balance of 800,000 is payable in twenty equal installments. B defaulted on the succeeding installments. S sued B for the collection of the balance and the court rendered judgment in favor of S. S by execution of judgment sold the car for P700, 000. Can S recover the deficiency? Give reason/s?