Is the mentioned transaction, which was carried out in the form of a Murabaha at a rate of 3% per month, and in which the Murabih (purchaser) became unable to pay after a period, valid and does it obligate the Murabih to the entire agreed-upon amount, or is he only obligated to the principal amount, keeping in mind that among the heirs are minors and adults?

The transaction mentioned is closer to a Mudarabah (profit-sharing partnership) between the capital provider and the agent. The former provides the capital, and the latter provides his effort and expertise. It is stipulated that the agreement must be on a common share of the profit for each of them. The loss is borne by the capital provider, unless there is transgression or negligence on the part of the Mudarib (agent).

If the Mudarabah is based on the Mudarib guaranteeing the capital, then it is legally invalid. In an invalid Mudarabah, the capital provider gets his capital and whatever remains of it, he gets all the profit, and he bears all the loss. The Mudarib is entitled to a fair wage (ajr al-mithl). This is because the profit is the growth of the capital provider's wealth, and the Mudarib deserves a share of it based on the condition, but the invalidity of the contract invalidates the condition. Thus, an invalid Mudarabah is like an invalid lease.

If the intention is Murabahah (resale with declared profit) with deferred payment, then the debt is the responsibility of the buyer (the Mudarib), and he must repay it to the seller's heirs.

It is preferable to present the issue verbally to scholars.