rob the till
rob the till or have one's hand in the till {v. phr.}, {informal}
To steal money in your trust or for which you are responsible.
The supermarket manager suspected that one of the clerks was robbing the till.
Mr. Jones deposited one thousand dollars in their joint savings account and told his wife not to rob the till.
The store owner thought his business was failing until he discovered that the treasurer had his hand in the till.
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Source: A Dictionary of American Idioms