What should a person do who stole 100 Saudi riyals from a company (Panda) 16 years ago, and now the ownership of the company has changed and it has become (Al-Aziziyah Panda), and he regrets it and wants to repent?

What you did by stealing money from the company is a sin and a betrayal of trust. Repentance from theft requires returning the stolen item or seeking forgiveness, as the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it took until it returns it."

The amount must be returned to the owner of the company at the time of the theft, not to whoever the company transferred to afterwards. If you are unable to reach him, then donate the amount on his behalf to the poor and needy, as Ibn Taymiyyah said: "If a person has in his possession usurped items, or borrowed items, or deposits, or pledges, and despairs of knowing their owners, then the correct action is to donate them on their behalf."