Is it incumbent upon one who purchased a sheep thirty years ago for a certain sum, and has not yet paid, to pay that same sum or its current value upon payment now?

If a person owes an old debt and the currency exchange rate has changed, he is only obligated to pay the agreed-upon amount of the debt, which in this case is forty riyals. If the debtor is in straitened circumstances, he is not liable for anything except repayment, as Allah Almighty says: "And if there is any debtor in difficulty, then grant him time till it is easy for him to repay." However, if he is capable and procrastinates, he is sinful and must repent, as the Prophet (peace be upon him) said: "The procrastination of a rich person is injustice."