I was just thinking, say you pay someone a thousand Euro a week. So maybe that's 1,132 dollars the first week of January, but with conversion rates changing, maybe for week 2 it's 1,129 dollars. You'd add them all up and that's their pay for the year for plan purposes.
If you pay them in dollars it's obviously easier. But the 415 regs don't exclude compensation based on the location of the services.
I don't know how the employee ends up taxed by either or both countries, though. Everything I peeked at started referencing the 800s and 900s of the Code, so I sorta bailed at that point.