What is the ruling on someone who took money illegally, repented, wants to atone for his sin, but does not possess the money?

Remorse over sin is the first step towards repentance. Taking people's money unjustly is a forbidden sin and transgression, and repentance is not complete until what was taken is returned to its rightful owners, as the Prophet (peace be upon him) said: "The hand is responsible for what it takes until it returns it."

What is required of you, in addition to remorse, is a firm resolve not to return to that [sin], to increase seeking forgiveness and performing righteous deeds, and to be determined to fulfill those rights whenever you are able, even if through installments.