How can a past debt of unknown amount be settled with shopkeepers who do not recall the debt, and is it permissible to give the money in charity on their behalf?

If the grocery store owners are unaware of the amount of the debt, then you must estimate it through investigation and caution, and each of you should forgive the other. If the worker refuses to accept this, communicate with the grocery store owner to pay him his due and explain the matter. Do not give these amounts in charity except when despairing of reaching their owners. If that is not possible, you may give them in charity on their behalf.