Am I accountable for anything if I made a mistake in writing the amount of a debt in a document of debts between two people?

If you made a mistake in writing down the amount of the debt, there is no sin upon you for the error. The debt remains what is actually owed by the debtor. Writing less or more does not change the reality of the debt. A judgment does not make permissible what is forbidden. Whoever is judged to receive something that is not rightfully his, it is as if a piece of fire has been cut for him. If the error becomes clear, you must rectify it and inform both the creditor and the debtor.