he discretion to grant an individual immunity from prosecution. We have held that "due process requires the prosecutor's promise to be fulfilled." Rowe ro. Gr(05n, 676 F.2d 524, 528 (11th Cir.1982); see also Plaster v. United States, 789 F2d 289, 293 (4th Cir.1986); United States v. Fountain, 776 F.2d 878,