Is it permissible to rebuke a vendor who deliberately shortchanges the amount due, and is that considered bad manners, or should one just take the money as if he truly forgot?

One should not harbor ill thoughts about others or accuse them without proof. If a person's deliberate intent is confirmed, they should be advised gently and kindly, and the impermissibility of wrongfully encroaching upon people's property should be explained to them, all while exercising wisdom and good counsel.