Is the younger brother entitled to demand a sum of money from his older brother that he had given him as aid, not as a loan, and is the older brother considered a defaulter if he requests to pay the amount in installments?

If the younger brother spent on his elder brother with the intention of a loan, then he has the right to demand the money back. However, if he spent with the intention of a gift and donation, he does not have the right to revoke it unless he intended to revoke it. In this case, the elder brother must repay the amount if he is able. If he is in straitened circumstances, he must be given respite until his situation becomes easier, as Allah Almighty says: "And if there is one in adversity, then [let there be] a respite until [a time of] ease." The elder brother does not incur sin by repaying the amount in installments if he is unable to pay it back in one lump sum. The younger brother must observe proper etiquette in demanding repayment and show deference to his elder brother.