Toward the end, Agent Carver pulls Nick from the edge, as he's pretending to be suicidal. This is the reason Nick is able to take the drug back from Carver, thereby making Nick's plan work like a charm. If Carver thought Nick was about to jump, why would he pull him back, saving him? Nick's plan won't work if this doesn't happen, since the injection is a fraud. Because of Carver's interference, Nick was able to wrestle the "drug" back from him.