Alan owes Brian HK$5,000, which is due on 31 March. On 15 March, he sought Brian's consent to accept a lesser sum of US$600 on 24 March (i.e., one week in advance of the due date) to discharge the whole debt. Brian agreed to this arrangement. Alan then made a bank loan of US$600 at an annual interest rate of 5% with a view to paying Brian. Today is 24 March, Brian has now changed his mind. He insists to revert to the previous position of full payment HK$5,000 to be paid on 31 March. Advise Alan if his part payment is able to discharge the whole debt in such case. [ Discuss the doctrine of promissory estoppel.]