are they docking a 'bonus' you recieve for reaching certain goals? or is it straight salary being docked?

where i live, 'incentive payments' are considered binding if the employer has agreed to pay them, and follows the agreement with an actual payment...

there should be an agreement in your personal file... what does it say there regarding customer complaints and pay docks?

that will be what a court looks at... black and whites on paper.... verbal, a bit more difficult to prove, but you can use prior pay records as evidence of agreements..
i.e. .. "this was the prior agreement X and the pay stub reflect that"

good luck...