Rad al-Madhalim is one of the most precisely structured obligations in Islamic jurisprudence and it deals with a very specific and serious situation: the wrongful taking or use of another person's property or wealth without their permission.
The general ruling is that seizing or using the property of another person without their permission is considered usurpation, which in Arabic is called Ghasb. It is obligatory for the person who did this to repent to God and to restore or reimburse what they took through a specific series of steps.
The first step is always to restore the usurped property or pay its full monetary equivalent directly to the rightful owner. If the owner is deceased, it must go to their inheritors. This direct restoration is always the first obligation and must be pursued before anything else.
If direct restoration is not possible for any reason, the second step is to restore or reimburse the amount indirectly to the rightful owner or their inheritors. An example of this would be anonymously mailing a cashier's check to the person. This step is only taken when direct contact or transfer is genuinely not possible.
If neither direct nor indirect restoration is possible, the third step is to give the usurped amount to the poor as Sadaqah on behalf of the original owner. However based on obligatory precaution this third step should only be done after receiving permission from the Marja or his authorized representative. You cannot simply decide on your own to skip straight to this step.
If the usurped amount is unknown and you are not certain of the exact figure, you are only required to pay the minimum amount you are certain of. For example if you believe the amount is either $100 or $150 but are not sure, it is sufficient to pay $100. However it is advisable to pay more to ensure the actual full amount is covered.
If a person is unable to pay the amount in full or even partially, they must pay whatever they are capable of and must designate someone to fulfill the remainder in the event of their death. The obligation does not simply disappear because of financial hardship.
Rad al-Madhalim is a profound expression of Islamic justice. It does not allow a person to hold onto what does not belong to them simply because time has passed or the original owner cannot be easily found. The rights of people over their property persist, and Islam provides a complete structured path to make those rights whole again.