Is it permissible to give as charity a sum of money that has not been fully repaid to its owners, due to the long period of time and forgetting the remaining amount, given that there are other debts to an absent person and to others who are present?

It is obligatory for anyone who participated in a trip or similar activity to share the cost, and for each of them to pay their full share. Whoever forgot to pay their full share is now obligated to do so, and the passage of time does not waive this right unless its rightful owners forgive it. As for someone who cannot be reached, charity should be given on their behalf for their share. It is permissible for a person who owes the questioner a debt to set it off against their debt if they are equal, or by the lesser amount if one of the debts is greater.