Is it obligatory to repay debts when money becomes available after years, or should they be negotiated or forgiven, or left as they are?

It is obligatory to fulfill rights and repay debts as soon as possible, for rights do not lapse with the passage of time. There is no blame upon the debtor in straitened circumstances to ask the creditor to forgive his debt, or part of it. Otherwise, the creditor's right is to postpone repayment until the debtor becomes solvent, as Allah Almighty says: "And if there is one in straitened circumstances, then [let there be] postponement until [a time of] ease. But if you remit [it by way of charity], it is better for you, if you only knew."