If I have a Roth IRA while living in the US and want to contribute to it after I move overseas to work for a few years, how do I do this? Say I make 70k overseas and plan to take the foreign earned income deduction. Can I choose to only deduct $64,500 (instead of all my foreign earned income) and pay taxes on the remaining 5.5k and contribute that to my Roth IRA?
You can contribute to Roth IRA regardless of the foreign income exclusion. You can contribute to Roth IRA only for amounts in excess to FEIE. However, the MAGI limit for Roth IRA contributions ignores the FEIE.
So in your case, if you exclude your income, you would not be able to contribute to the IRA.
You cannot partially exclude the income. You either take the full exclusion, or none. So instead of exclusion, what you can do is chose to take a Foreign Tax Credit to reduce your US tax liability, and then you'll be able to contribute to the IRA as you wish.
Here is an article explaining that you can contribute to your Roth or Traditional IRA by taking a Partial Foreign Earned Income Exclusion:
@littleadv says, "You cannot partially exclude the income" but someone has take enough time to author the article I have referenced and I am seeing logic in a partially excluded income filing. When would the IRS ever prevent you from claiming income? Also, a TurboTax algorithm not allowing that kind of nuanced tax form entry is not an authority on the situation either possibly...
However, here is another article that supports @littleadv's answer: http://www.crevelingandcreveling.com/blog/62-american-expats-and-iras-a-how-to-guide.html