The exception to the rule that past consideration is not good consideration is subject to conditions discussed by the Privy Council in Pau On v Lau Yiu Long, wherein their Lordships held that this was admissible whenever:
Whether, part payment of an existing debt is valid consideration for a promise has been decided in the Pinnel's case. In this case, Cole owed Pinnel 8-10s-0d (8.50) which was due on 11 November. At Pinnel's request, Cole paid 5-2s-2d (5.11) on 1 October, which Pinnel accepted in full settlement of the debt. Subsequently, Pinnel sued Cole for the amount owed. It was held that part payment in itself was not consideration. However, it was held that the agreement to accept part payment would be binding if the debtor, at the creditor's request, provided some fresh consideration (Pinnel's Case,1602).
From the foregoing it can be concluded that, under certain circumstances, past consideration is held to be valid. ...