Syria are getting billed, at some point, for all the tanks Russia dragged out of their reserve to supply them. The T90 seem to have cast turrets, so are from the initial batch before Russia ran out of money and the ability to do large castings. Confusingly, the Russians are using Syria as a giant arms advert with systems used only by Russian personnel also showing up; SU-57 anyone?
The Iranians have lent them billions with no prospect of getting any of it back, and the Russians are carpet bagging the Syrians with offers of reconstruction work, but the combination of War and sanctions means that there is no finance in place for it.
A big, shiny multi-million missile system would be an enormous purchase. So the Russians would have to give it away.