there is an expired tratry and an international agreement.Iskander has 500km range. So no, the range limitation for ballistic missiles was not 300km.
The INF restricted the range in 500 km, means Russia can't have land launched ICBM or crusie missile (drones) capability.
The other is
It is a non-binding one, but the signaturing countries restricted the technology sales up to this date by the restrictions of this agreeement.
Due to the later one the Russian missiles, including the developed in cooperation, like the BRAHMOS don't have more than 300 km range.
Iran si NOT a signatory of this agreement, but USA/Russia up to this date followed the restrictions of it.
Means if the USA ship more than 300 km range missiles to Russia then Washington effectivly breaking the rules developed on his own.