In their letter to Geither, the two senators noted that the amendment allows the president to waive sanctions against foreign financial institutions if their home governments have significantly reduced the volume of crude oil purchases from Iran. The senators called on Geithner to meet that standard by focusing on the reduction of purchases of Irans oil, as that is most likely to affect the nations access to currency.

In the end, a successful sanctions policy will be judged by its impact on the Iranian bottom line — driving its net oil revenues down regardless of its actual supply of oil to the market, they wrote.

They asked Geithner to worry less about whether imports from Iran have been significantly reduced, as this could allow a broad range of criteria to be considered that could weaken the sanctions.

The senators wrote that they understand the Treasury is currently writing regulations to implement the amendment, and that they hope their letter conveys the amendment’s legislative intent.