Shift and hours exchange

An employee A in need to skip a shift our a few hours, can request to all other employees, or a certain kind of them, to cover him/her, indicating the date and hours, (maybe also the position, job, and reason for the absence). Another employee, say B, willing to take over, can do so. (The replacement might need to be authorized by the employer, or not.) Each employee has an account if worked extra hours, and due hours, that s/he as well as the employer can check out anytime.