The UN Resolution applying sanctions to Iran does not prohibit the sale by Russia of the S-300 anti-aircraft system.
In the text of text of resolution 1929 (2010):
8. Decides that all States shall prevent the direct or indirect supply, sale or transfer to Iran, from or through their territories or by their nationals or individuals subject to their jurisdiction, or using their flag vessels or aircraft, and whether or not originating in their territories, of any battle tanks, armoured combat vehicles, large calibre artillery systems, combat aircraft, attack helicopters, warships, missiles or missile systems as defined for the purpose of the United Nations Register of Conventional Arms, or related materiel, including spare parts, or items as determined by the Security Council or the Committee established pursuant to resolution 1737 (2006) (“the Committee”), decides further that all States shall prevent the provision to Iran by their nationals or from or through their territories of technical training, financial resources or services, advice, other services or assistance related to the supply, sale, transfer, provision, manufacture, maintenance or use of such arms and related materiel, and, in this context, calls upon all States to exercise vigilance and restraint over the supply, sale, transfer, provision, manufacture and use of all other arms and related materiel;
Note the reference to the Register of Conventional Arms
VII. Missiles and missile launchers (in the Register Categories)
Guided or unguided rockets, ballistic or cruise missiles capable of delivering a warhead or weapon of destruction to a range of at least 25 kilometres, and means designed or modified specifically for launching such missiles or rockets, if not covered by categories I through VI. For the purpose of the Register, this category:
(a) Also includes remotely piloted vehicles with the characteristics for missiles as defined above;
(b) Does not include ground-to-air missiles.
Read that again. The resolution does not include ground-to-air missiles.