Under the PAYE income-driven repayment plan, the monthly payment is based on the borrower's Adjusted Gross Income (AGI) on his or her federal tax return. If you are filing married filing jointly (MFJ), the AGI will include both of your incomes, but if you file married filing separately (MFS), her AGI will not include your income.
The catch, of course, is that filing MFS will almost certainly result in a higher tax burden for the both of you than MFJ would.
If you file MFJ, it is possible that your combined income level will make you ineligible for the PAYE repayment plan, as the resulting monthly payment would be no less than the standard 10-year payment amount.
The idea of loan forgiveness is nice, but keep in mind that 20 years is a long time. During the whole time, you will need to worry about keeping your wife's income low (so you don't pay too much) and you need to file MFS, paying extra taxes each year.
An alternative for you is to simply decide that once you are married, you will pay off this student loan in full as quickly as you possibly can. With your combined income being well into the 6 figure range, you could conceivably pay off this loan in full in perhaps 3 years without much difficulty at all. You can file MFJ the entire time, saving money on taxes, and three years from now the debt will be out of your life and you'll have the full earning potential of both of you without worrying about whether or not it will mess up your student loan plan.
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