How can one repent from theft and restore injustices, especially if the stolen items have been consumed or their owners are unknown?

If a thief repents, but is unaware of the rightful owners or the exact amount stolen, then let him give in charity what he has of the stolen goods or their value on behalf of their owners. He must strive to estimate what he stole in order to clear his conscience, for Allah does not burden a soul beyond its capacity.