> Under federal law, the general rule is that employers may deduct certain expenses from their employees' paychecks, as long as the deductions don't bring the employee's earnings below the minimum wage
> Some states prohibit paycheck deductions for debts to the employer, or limit the circumstances under which these deductions may be made. For example, state law might require employers to secure the employee's agreement, on a signed consent form, to withhold this money.
The agreement on the website isn't a signed document. Anyway, there are much easier ways to distribute the bonus: just pay once it's achieved.