If the person who lost the wallet is known, should the full amount be returned to him, or only what was taken, especially since the others have forgotten what they did? And if the person is not known, what should be done with the money?

The ruling for this money is the same as that for a lost item (Luqata). The person who found it must announce it for a year in the place where it was found. If its owner comes and identifies it, it must be handed over to him. It is not permissible to take it and dispose of it, and whoever does so becomes liable for the lost item.

One must repent to Allah and announce the money. If the owner is not found, the finder has the choice between keeping the amount or giving it in charity on behalf of its owner. If the owner appears afterwards, he has the choice between accepting the charity or taking the money.

Liability is obligatory to the extent of the finder's share of the money. If he was the sole finder, he is liable for the entire amount. If he shared with others, he is only liable for his share. He must advise his partners and remind them of the obligation to repent. Entitlement to a lost item is acquired by taking possession of it, not merely by seeing it.