My two cents on sales of S-400 anti-aircraft defence by Russia to Saudi Arabia in October 2017 which are today on their way to Saudi Arabia and helped Russia circumvent sanctions:
I think but am not sure, that Saudi made a deal to let Russia circumvent sanctions and have money etc as part of a US-Saudi plan.
The reason is that the S-400 are set by Russia in Syria and control a big radius of air.
Since U.S. can’t access them it could have planned the purchase by Saudi so that U.S. can study their technology and find a counter-technology.
I could be wrong but U.S. didn’t protest their sales, unlike the case of Turkey.
More to the Russia – Saudi deal under this article: