"Charlie borrowed $100,000 from Ah Long to help his son through university education. The payment would be by instalment, starting from the moment Charlie's son has completed his university education. Ah Long was most impressed with Charlie son's progress in his undergraduate studies and decided to give John a discount. John needs to pay only $60,000 which he had paid it in full. A few months later, Ah Long changed his mind and wanted Charlie to repay the full loan, amounting to $100,000. Advise Charlie on what he should do."

Would this case study be in relation to applications of Promissory Estoppel? Thanks for your help!