Why else would it have needed to be in non-US-dollar cash?
It should have been a simple $400m ACH. Why wasn't it?
Obviously, because it violates US law in some fashion Zero's Regime feels compelled to hide.
Zero's survival as President is a matter of two thirds of the Senate. Would it have been possible to muster two thirds of the Senate over whatever law he would have been violating by using an ACH?
No? (Duh!) So, why did he bother?
Why cash at all! The type of currency used shows that the law is stupid, it has no effect but to cause people (who want to follow the law) to do a currency exchange. The point of the law is to curtail payment of tribute or ransom, not to cabin the types of curency that are permissible when paying tribute or ransom.
And to the category of "ransom" or "settlement of a debt," the administration would have you leap to a conclusion that a payment can't be both. If release of the prisoners was contingent on the payment, then the payment was ransom.