I work for a small lumber yard in Maryland and a cusstomer got some material and didn't pay for it,my employer in turn charged the material to me,can he do that ?

Patty should be able to tell you for certain but in most states no, he would not be able to charge you for mistakes.

No, they cannot. They may discipline you in other ways for not following procedure or exercising due diligence, up to and including termination, but they cannot affect your pay.