It is normal in official sales of advanced arms that the buyer commits not to transfer or re-sell the systems to another party, for otherwise the seller loses control and the arms might reach an adversary. Belarus is close to Russia politically, and even depends on it in many ways. It is simply unthinkable that they would have transferred the missiles to Iran without Russian authorization. So here is how it worked: The Russians had a contract to sell the S-300 to Iran. They were pressured by the West not to deliver (even though this system is not included in the UN sanctions). For not delivering, the Russians were going to obtain some benefits from the West. On the other hand, the Iranians are good clients and it was important to keep them happy. What do you do? The compromise: Russia secretly allows Belarus and one or two more very good friends to transfer a small number of batteries to Iran. The Iranians are appreciative enough and Russia still keeps the option to sell more if the West doesn't deliver benefits. Incidentally, the US under Reagan authorized Israel to sell US missiles to Iran in order to overcome Congressional sanctions...
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from the article: Iran official: We have obtained the S-300 missile system