Can we deduct from an employee’s paycheck if an advance isn’t repaid?

Q: “Can an employer legally deduct ‘reimbursements’ owed by an employee via the employee’s paycheck? If so, what process or forms need to be in place? When is it never allowed? For example: A company lends money to an employee in an emergency as a loan, and has the employee sign an agreement to pay back such loans.” – Chuck, Illinois