Is an installment sale made by the father, of which outstanding installments remain and the buyer has paid a portion, considered a usurious sale? And what is the correct Sharia-compliant action for the heirs in this situation?

You must refer to the judiciary to rule on matters of inheritance and common rights, especially if there are minor orphans among the heirs, as their share must be preserved and not waived. As for the issue of "da wa taajjal" (stipulating a reduction in debt upon accelerated payment), the majority of scholars prohibit it, while some, like Ibn Taymiyyah and Ibn al-Qayyim, permit it. We believe there is no objection for the seller to accept this transaction.