Since one presumes that Microsoft does not sell much music (didn't they shut down their music store a few years ago?), I can't imagine they've negotiated dues that scale linearly on Microsoft's total revenue - so, in fact, I'd expect that $1M is well beyond their RIAA dues.

Now, it's possible that MS is contributing money other than dues to the RIAA, but you can bound that a bit from their Form 990: https://projects.propublica.org/nonprofits/organizations/131... For 2017, they got under $100K in "contributions and grants" and $29MM in "program service revenue," which is later determined to be dues. They received no other significant revenue. So no donor could have possibly given them more than $100K.

(... also, why did the RIAA give $4K to the Kenai River Sportfishing Association?)