Unfortunately there is no way for the IRS to recognize the divorce agreement. The tax liability was a joint liability of $18,000. The IRS is not going to sever that debt because of the divorce agreement. The IRS will collect from whichever party they can. If the IRS forces you to pay his agreed upon portion of the debt, your only recourse is to pursue him legally under that divorce agreement. Pursuing him through the courts will be ineffective since he lives in another country.