Old news. The Obama administration negotiated the Iran nuclear deal, along with other world powers, which involved lifting sanctions in exchange for Iran limiting its nuclear program. Un-freezing assets was part of the agreement. It was their money. And the money really never mattered,... like the U.S., they just printed more.
Certification that Iran is complying with the deal must be sent to Congress every 90 days.
The first certification under the Trump administration noted that Tehran was in compliance.