An investigation by Senate Republicans released Wednesday sheds light on the delicate balance the Obama administration sought to strike after the deal, as it worked to ensure Iran received its promised benefits without playing into the hands of the deal’s opponents. Amid a tense political climate, Iran hawks in the U.S., Israel and elsewhere argued that the United States was giving far too much to Tehran and that the windfall would be used to fund extremism and other troubling Iranian activity.

The report by the Senate Permanent Subcommittee on Investigations revealed that under President Barack Obama, the Treasury Department issued a license in February 2016, never previously disclosed, that would have allowed Iran to convert $5.7 billion it held at a bank in Oman from Omani rials into euros by exchanging them first into U.S. dollars. If the Omani bank had allowed the exchange without such a license, it would have violated sanctions that bar Iran from transactions that touch the U.S. financial system.

The effort was unsuccessful because American banks — themselves afraid of running afoul of U.S. sanctions — declined to participate. The Obama administration approached two U.S. banks to facilitate the conversion, the report said, but both refused, citing the reputational risk of doing business with or for Iran.