The Fed will credit the receiver’s account even if the sender doesn't have enough money of its own at the time of the transfer. This is an element of what the Fed calls ‘finality.’ Once its account is credited by the Fed, the receiver (Citibank, for example) is insulated if the sender (say, Chase) does not have the funds at the end of the day. If Chase can’t pay, however, the U.S. taxpayer effectively will.