Will I be sinful if I unintentionally make a mistake when writing down a debt amount for someone, and does the Hadith "Indeed, Allah has pardoned for my Ummah their mistakes and forgetfulness..." apply to me?

There is no sin upon one who errs in writing the amount of a debt due to an excusable mistake, and what matters is the amount actually owed by the debtor. Writing a lesser or greater amount does not forfeit the remainder or establish the excess. If the error becomes clear, you must rectify it by informing the creditor and the debtor. You are not liable if either of them exploits the error, as you are entrusted and not negligent.