When a taxpayer asks the IRS to eliminate their tax debt through an offer in compromise, the IRS will look at the persons assets and past income to determine if there is a likely chance that the taxpayer will be able to pay back the total tax owed. In some cases, if the taxpayers income history has been variable, the IRS will want to get a portion of the future income above certain thresholds. This is known as a future income offset agreement. It is not a typical request but it is sometimes part of the settlement with the IRS. The good news is that this may cause the deal to be accepted by the IRS, so it is worth considering.
While this arrangement has certain complexities, the overall idea is that the taxpayer may be able to afford more in the future than what they show they can afford today (on a form 433(OIC), and within certain limits, that they can pay more than the total amount offered if it was paid overtime. While this money is not a certain guaranteed payment to the IRS, it may cause them to be willing to accept the deal with the hope that in the future they may get more. From my experience, the IRS will only look to the next few years, so it would not be the case that five years from now they are still trying to get money from you.
The taxpayer is looking to make a deal to settle their tax debt, so often these additional monies being paid to the IRS is workable to most people. As an example of this concept, say a person owes $100,000 in back taxes (including interest and penalties). They may be currently working and making $60,000 a year from their wages. They have a 401(k) retirement plan that is worth $20,000, but that after taxes are paid they will net about $11,000. It is clear that will not be able to pay back the whole $100,000 owed since the $60,000 in wages only covers normal living expenses. Say the person offer the value of their retirement account of $11,000 plus an additional $4,000, for a total offer of $15,000. It may be that the IRS accepts such an offer if it was made by someone in their sixties, but they would not if the person was forty years old. In the case of the forty year old, the IRS may accept the lump-sum of the $15,000 and then ask for twenty percent of the monies earned for the next three years greater than the wages of $60,000 a year. If the person makes, for instance, $80,000 for the next three years, the IRS would receive the initial $15,000 plus $12,000 ($80,000-$60,000 times 20%), or $27,000, in total.