The biggest depositor

A new teller at the bank found that a colleague was embezzling enormous sums of money. He went straight to the manager and told him, "Mr Smith is robbing us of thousands of dollars."
"Yes," said the manager. "I know."
"You know?" gasped the teller. "Then why don't you fire him?"
"We can't afford to," sighed the manager. "He's our biggest depositor."