I'm assuming that this is based on some claimed national security threat else the FISA court would laugh.
So what happens when Mexico says "OK, here's the money?" Do we let the remittances resume, thus making it clear that the claimed threat was a sham? If not, what's Mexico's incentive to negotiate?
Drug cartels and terrorists are crossing the border with impunity; Mexico is not cooperating with the US in stopping them. Affect the billions of dollars in remittances and they’ll see the light. They’ll get on board.
I’m not sure if the FISA court is the right one, but similar I suppose.