Accountant in bank

Two accountants are in a bank, when armed robbers burst in. While several of the robbers take the money from the tellers, others line the customers, including the accountants, up against a wall, and proceed to take their wallets, watches, etc. While this is going on accountant number one jams something in accountant number two’s hand. Without looking down, accountant number two whispers, "What is this?" to which accountant number one replies, "it’s that $50 I owe you."