Reza Zarrab's partner in an elaborate sanctions scheme, the Iranian billionaire businessman, Babak Zanjani*, was sentenced to death in Iran, earlier this year, on corruption charges; Zanjani was funneling hard currency to Iran, to evade the international financial sanctions, and Iran claims he owes the state $2.8bn. There are indications that the death sentence might be modified, to a term of imprisonment, should he pay over the funds.
Inasmuch as Zarrab, and other individuals with whom Zarrab and Zanjani are partners, have access to the money claimed by Iran, the country's judicial authorities, and officials, have made no secret of the fact that they want Zarrab, and the money, in their hands. What effect this will have on future exchanges of prisoners is not known.
* Named a Specially Designated National (SDN) by OFAC in 2013.