You may call 1-800-829-1040 to modify or terminate your agreement.
You also may modify your payment amount or due date by going to www.IRS.gov clicking on “Tools” and then “Online Payment Agreement.” according to the instructions at http://www.irs.gov/pub/irs-pdf/i9465.pdf
If the new payment amount will not fully pay the tax within five years it is more difficult; but it is common for the amount of an installment agreement to be modified to less than what will fully pay when circumstances change that would not allow you to make the initially agreed payments without hardship.
Please understand, though, that is it not actually a negotiation; but rather the amount is based on the IRS formula and standard expense amounts that determine what is available for payment so data will need to be submitted about your financial situation.
If you are in danger of defaulting on your payment agreement for any reason, it is best to contact the IRS immediately.
Please ask if you need more information or clarification.
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