Is eating from another person's share unlawful if the original owner did not eat it, and what is sufficient repentance for that, and is it necessary to seek forgiveness from the organization's owner, who is difficult to reach?

If there is an agreement on a specific portion of food, it is not permissible to take an excess amount without the owner's permission, because consuming people's property unjustly is forbidden. Allah Almighty says: (Do not consume your property among yourselves unjustly, but [instead, let there be] trade [conducted] by mutual consent among you). And the Prophet, peace and blessings be upon him, said: (The property of a Muslim is not lawful except with his willing consent).

Not eating on some days does not permit taking an excess amount, because the deficiency is from you.

What is obligatory is to repent to Allah and return the value of what was taken, or to inform the person in charge of the food not to take meals to compensate for what was taken. It is not necessary to ask the owner for forgiveness; rather, returning the right to him is sufficient, even if he does not know. If direct notification is difficult, the right can be delivered indirectly, as mentioned by Sheikh Ibn Uthaymeen.