Since the employee payroll contribution is technically a voluntary salary reduction with the employer making the deposit, they can make any rules they want.
However, you can take your own cash and make a deposit, then collect the tax savings at the end of the year instead of getting the savings in each paycheck.
Or, if you can do 10 equal payments of $650 for the rest of the year, you can pay the bill now with other money and take the $6500 back out in December. You don't have to pay the bill only with the account. You just have to ensure that the amount you withdraw is the same as the amount of bills you paid.
