If this happens to a bank (which rarely if ever happens, because few things in IT are as secure as banks), then the transfer can be traced, blocked, and reversed. Since the system is made up of points of control (aka. "gatekeepers"), and all flow, including withdrawal from the system (cash or spending) goes through gatekeepers, there are alot of points of control at whch flow can be controlled, analyzed, and even stopped.
Gatekeepers are not an evil to be overcome, they are an integral part to the security and stability of complex systems.