they wanted the suit just to further embarrass him by leaving him to die in the desert naked and alone. When Frank attacked, he got the shank, which actually just made his last few moments more merciful, with a shorter walk to his final collapse. The Mexicans missed an opportunity with the diamonds, but besides that they got a million cash and killed their rival. That was the plan. There was no scenario where honor or anything else was going to leave Frank alive. Frank tried to bargain, asking for a ride, then they asked for the suit, just to humiliate him further. He was never emerging from that desert alive.