What are the necessary conditions for repentance, and is it obligatory to fulfill a promise or is repentance sufficient, in the event that one denies a promise to pay the price of a commodity after the promised party has purchased it, and this falls under "when he promises, he breaks it"?

Scholars differ concerning the implications of a promise. If the promisee, due to the promise, enters into a commitment, then fulfilling it is considered recommended by the majority of scholars, but obligatory according to the Malikis. Accordingly, you must fulfill your promise to this person regarding the price of the watch, according to the Maliki school, because he proceeded with its purchase due to your promise. Repentance is not complete except by fulfilling what is obligatory. As for other schools, fulfilling it is recommended. Your denial of your promise to him is a lie, from which repentance is obligatory.