What is the ruling on a promise of sale if the promisor dies before the completion of the sale and the heirs refuse to complete it?

The promisor is not obligated to sell, and fulfilling a promise is desirable. The promisor does not sin by breaking it, but rather misses out on a virtue and commits an act that is severely disliked (makruh). Therefore, the heirs are not at fault for refusing to enforce their deceased's promise.