Is money sent from a brother considered aid or a debt, and do his heirs inherit this debt after his death, and how can the value of the due debt be estimated if the original amounts are old and their price has inflated?

If someone asks for money and is given it, and the one who asked claims it was charity while the one who gave it says it was a loan, then the word of the giver is to be taken. If the receiver did not claim to be poor, then the money is a debt upon him. A debt is repaid in kind, not by the changing value of the currency, as long as the currency exists; meaning, the same amount that was taken is to be repaid.

It is stated in a resolution by the Islamic Fiqh Academy (Majma‘ al-Fiqh al-Islami): "The consideration in repaying fixed debts in a certain currency is by its like, not by its value, because debts are repaid with their likes."

If you increase the repayment to account for inflation, that is good, based on the saying of the Prophet (peace and blessings be upon him): "Indeed, the best among you are those who are best in settling debts."

This debt is included in the inheritance and is divided among the heirs according to their prescribed shares.