Is it incumbent upon someone who purchased a car through a lease-to-own system in his uncle's name, and undertook to pay all installments himself, to pay the remaining installments to the heirs after the uncle's death and the waiving of the installments?

It appears the questioner authorized his maternal uncle to carry out an Ijarah Muntahiyah Bittamleek (leasing ending in ownership) transaction for a car. Then, the uncle passed away, and the owning entity waived the remaining installments. The uncle's heirs have no right to these installments because their waiver is compensated from a special fund for death cases, and contractual rights pertain to the principal (muwakkil), not the agent (wakeel). Moreover, ownership transfers directly to the principal upon purchase by agency. Even if the rights were related to the agent, that would be during his lifetime, not after his death. If the waiver of installments was conditional on the death of the actual lessee, then the questioner should clarify the matter to the leasing entity. If they demand payment, he must complete them.