As long as the employee signs and agrees to the deduction, you can do it. However, the Federal Labor Laws specifically state that you can NOT take their wage below "minimum wage" even if they agree to the deduction.
In the same vein... if the employee damages something and then agrees to pay for it (before or after the incident), you cannot take the paycheck below minimum wage, no matter what. Feds will make you give it back IF you get audited. BTW - the federal statute of limitations is 2 years on a wage audit. In other words, they can only go back two years to check to see if you were kosher in your dealings with regards to wages.