What should one do who found a sum of money at his workplace, spent it, and did not deposit it in the lost and found office?

The money your friend gave you is considered a lost item (Luqata). It must be publicized with the intention of returning it, not with the intention of owning it. It is permissible to entrust it to a trustworthy person like yourself to publicize it. If you dispose of the money and take it for yourself, this is treachery and wrongfully consuming wealth. You must repent and deposit the amount in a reliable lost and found office. Otherwise, you are obliged to publicize the money while guaranteeing it. It is forbidden for someone who does not trust themselves with a lost item to take it, and they must guarantee it if it is lost. It is obligatory to return it to its owner or hand it over to the judge.