There is no doubt that even though sanctions forbid selling certain specified goods (including gasoline) to Iran; it is perfectly legal to purchase Iranian goods. Because all dollar transactions are cleared thru New York, no payment can be made in dollars. Additionally, majority of the banks do not wish to engage in Iranian business because of the fear that their branches in the USA would be closed.
Like it or not, truth is that ever since the revolution, India has had lot more political clout with Iran. Among other things, one of the main reasons was that Pakistan supported Taliban whereas Iranian supported Northern Front in Afghanistan.
In August 1998 following the fall of Mazaar Sharif, Taliban troops; in addition to butchering of 8000 Shia Hazaras including women and children; also killed 11 Iranian diplomats. This was clear violation of the International Law of the Diplomatic Immunity.
Iran mobilized 70,000 troops on Afghanistan border. Taliban were then much loved by the ISI and probability of Pakistan coming out in support of Taliban and fighting Iranian Army was quite high. Better sense however prevailed and thru Nawaz Sharif diplomacy, Taliban agreed to return bodies of the slain diplomats to Iran. This defused the crisis.
I speak to NIOC about three /four times per week. Please take my word for it; Iran will never cut India’s petroleum supplies.