Sanctions on Iran began in 1979 with successive waves from administrations and Congress. They form a tight web of statutory and executive sanctions that cannot be undone at the drop of a hat. Trump can unilaterally waive sanctions for 60 days (as with the general license last week). However, allowing Iran to accept payment for oil in USD requires a US or US-linked bank to play a correspondent role, and banks are reticent due to uncertainty about where this will land.

Banks don't want to be left holding the bag if Congress or another administration investigates later.

Lifting sanctions would make a huge difference to the Iranian economy, particularly oil sales (the majority of government revenue). However, undoing them may require an act of Congress. For example, legislative sanctions require all US-listed companies to report any business touching Iran to the SEC, so Congress will have questions if things change.