It can be settled through negotiation. You can communicate and negotiate with each other, emphasize their previous commitments and ask them to fulfill their refund obligations. Disputes can be settled through friendly communication, but not through negotiation. You can send a formal letter (preferably a registered letter) to the other party, detailing the incident, the agreement between the two parties and the failure to perform the contractual obligations, and asking them to return the corresponding funds within a certain period of time.