The amount of diesel being provided by Russia in this deal is insignificant.
So, what is the real motivation behind this move?
All you have to do to figure out the motive is think how a petulant brat would act. Trump wants a win, this looks like a win to his moronic base despite it being fuck all and from a country actively helping your enemy. He is mad Zelensky did not stop attacking russian refineries as Trump appears to indebted to russia for some reason.
It actively helps russia with monetary benefit.
Nevermind that russia is currently in a fuel shortage and that you will never see a drop of the oil, it's all optics and bullshit.
You have an absolute moron in charge with zero plan and the impluse control of a crackhead.