How to input a positive balance when reconciling

It depends on the Account type:

Cash/banking accounts normally assume the amounts are positive. You shouldn't have to sign the amount.

Credit Cards (which I suspect you are dealing with) show the transactions/balance as negative amounts, but you enter the amounts unsigned (appearing positive) like they are on the paper statements.

If this is a credit card account with a truly positive balance (such as from a credit), then enter a negative amount for the balance, This isn't normal, but will make the number used positive for calculations as it will be opposite of what the program is expecting.