Never cash a cashier’s check where you are asked to return part of the payment.
Under no circumstances should you ever attempt to cash a cashier’s check where you are asked to return part of the payment. This is a classic scam, so please do not fall for it. There is no good reason why a legitimate employer wouldn’t be able to pay you the correct amount owed. Cashier’s checks are routinely forged and an “overpayment” check will often bounce after you send back the requested portion of the payment.