Right. If it hasn't cleared then start with what Opus 17 said: call your bank and see if they will honor the numerals you wrote.
If it has already been cashed for the lower amount then you can wait for a letter from the IRS requesting a payment for the difference plus an amount of interest they will specify (it won't be much) or you can go to the payments website Opus referenced and pay the difference.
If it hasn't cleared yet but the bank says they will go only with the lower amount, I wouldn't stop payment. I would go to the payment website and make a payment for the difference.
