I've dealt with some related issues to this, but in the EU jurisdiction(but am familiar with US landscape).
1: You are not prohibited from working with Russian developers who currently reside in Russia. The biggest issue is figuring out a way for you to pay them, but that sounds like its being taken care of by the company.
Assuming you don't decide that this is too much hassle (Even if you're not doing anything wrong, it sounds like your client would be. Do you really want to be part of that?) you need to talk to a lawyer who specializes in this sort of law.
Seriously though, you don't need this hassle.
java-man
You'll be funding the russian war in Ukraine indirectly (with the taxes paid by these developers).
1: You are not prohibited from working with Russian developers who currently reside in Russia. The biggest issue is figuring out a way for you to pay them, but that sounds like its being taken care of by the company.
2: You ARE prohibited from exporting consulting services to Russia, to either physical or legal persons, for "enterprise management software and design and manufacturing software": https://ofac.treasury.gov/faqs/added/2024-06-12#:~:text=1407...
3:Just contact OFAC. They are responsible for enforcement, and they have a hotline you can use.
https://ofac.treasury.gov/contact-ofac