"Oil is priced in US dollars, and bypassing the greenback posed challenges for both parties. Two banks are reportedly involved in this deal: India’s state owned UCO Bank and Turkey’s state owned Halkbank.
Both banks don’t have any business with the US and therefore are less vulnerable to sanctions. According to the report, an Indian delegation has spent time in Tehran and finalized the details of the transactions."
Iraq once stopped using greenbacks too for the Euro. That mess still hasn't cleared up.