Can a Business do this to it's sub contractors?

My dad worked for a flooring store about a year ago. he is a sub-contractor through them. They held 10% out of all his checks from customers until they had $1500 in what they called an "escrow" account. Even though he had is own license, insurance, LLC, and tax exempt. How long can they legally hold this money for? They said up to one year after the last day of employment. What do you think?