can he just get away with that what could be done is it worth the effort?

heres the story a friend of the family owns an a/c repair company,and he replaced a part for a guy.he left him the bill.a week or so latter he gets payment in the mail accompanied by a letter stating he would only pay X amount,which was maby $75 dollars short the amount.apparently he researched the price of the part.and took it upon himself to set how mutch hed pay for the labor not to mention mark up on the part.i mean sevice men buy wholesale where supposed to mark up you know.i know for a fact hes cheaper than we are so he didnt over charge.there is one catch hes a lawyer.you think he did this because he thinks that would intimidate him or maby to take advantage of a foriener.anyway does anyone know what could have been done.would there have been a point other than justice.i mean time is money you know.