A USNA may have to chime in with the exact details but basically the academy request an initial deposit to offset the cost of issue items, uniforms, computers, books, etc. It was $2,000 the last few years at USMA. If the cadet is unable to make the deposit before reporting their cadet pay will be reduced for until the amount is paid.
All cadets have a cadet account that is used to pay for required items at the academy. Money goes in and out each month. The account is designed to cover everything that is needed over the four years starting with the initial issue all the way to uniforms need when they are commissioned.