Let's face it - everyone makes typos. If you are your customer entered a check into the system with mistakes (wrong amount, etc) and you delete that check and try to reenter it using the same routing number, account number and check number - you will discover the system won't let you do that.
This is a security measure to prevent duplicate checks being created and processed through the account holders bank.
At this point, your only option is to contact your customer and ask for a new check number - or use one above 10000 and let your customer know. Check numbers are just for tracking and as long as its not a duplicate of a previous check - you are good to go.
