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To request an installment agreement - you should send Form 9465, Installment Agreement Request. You will be charged a onetime user fee of $105.00.
The IRS has a different procedure if you have more than $25,000 in combined tax, penalties, and interest may still qualify for an installment agreement, but a Collection Information Statement, Form 433F may need to be completed.
To initiate a payroll deduction installment agreement, submit Form 2159, Payroll Deduction Agreement. Form 2159 must be completed by your employer. IRS will set up a regular installment agreement for you and convert it to a payroll deduction agreement upon receipt of the completed form from your employer.
Another option is an Offer in Compromise (OIC).
An offer in compromise (OIC) is an agreement between a taxpayer and the Internal Revenue Service that settles the taxpayer's tax liabilities for less than the full amount owed. Absent special circumstances, an offer will not be accepted if the IRS believes that the liability can be paid in full as a lump sum or through a payment agreement. You should use the Form 656, Offer In Compromise The OIC fee is $150. Please be aware that the IRS issued a consumer alert advising taxpayers to beware of promoters' claims that tax debts can be settled for "pennies on the dollar" through the Offer in Compromise Program.
In most cases, the IRS will not accept an OIC unless the amount offered by the taxpayer is equal to or greater than the reasonable collection potential (RCP). The RCP is how the IRS measures the taxpayer's ability to pay and includes the value that can be realized from the taxpayer's assets, such as real property, automobiles, bank accounts, and other property. The RCP also includes anticipated future income, less certain amounts allowed for basic living expenses.
Negotiation of an Installment Agreement or an offer in compromise is often difficult and requires substantial knowledge about IRS guidelines and regulations - and most likely you need someone to represent you with the IRS.
I suggest contacting a local CPA or Enrolled Agent and he/she will evaluate your situation and estimate a reasonable offer.
Let me know if you need any help or clarification.