Have you tried using a promotion for the free shipping and setting a Promotion Requirement of a Shipping Country Requirement = United States?
You would need to turn off your current Free Shipping (I am assuming you have this setup via AppConfigs) and setup the promotion instead. The free shipping promotion also enables you to select specific shipping methods and a discount amount (100% off of shipping is what I think you want, but you could also discount shipping by a fixed dollar amount).
I am not sure how the discount will be displayed to the user during checkout without testing it out, but this may be an option worth trying out.
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