The United States v. Job L. Barber was a Supreme Court case in which the defendant, Job L. Barber, was accused of violating an act of Congress by selling goods to British subjects during the War of 1812 without obtaining a license from the President or Secretary of State as required by law. The court found that although Mr. Barber had not obtained such a license, he had acted in good faith and with reasonable cause to believe that his actions were lawful under existing laws and regulations at the time; therefore, they held him not guilty on all counts. This decision established an important precedent for future cases involving violations of federal statutes: even if someone has violated a statute unknowingly or unintentionally due to their own misunderstanding or misinterpretation, they may still be acquitted if it can be shown that their conduct was done in good faith and with reasonable cause for believing it was legal at the time it occurred.

In the United States v. Job L. Barber, Justice Johnson delivered a dissenting opinion in which he argued that the conviction of Barber should be overturned because it was based on an unconstitutional law. He argued that Congress had exceeded its authority by passing a law prohibiting citizens from trading with foreign nations without first obtaining permission from the President or Secretary of State, as this power was not delegated to them under Article I Section 8 of the Constitution. Furthermore, he noted that even if such a delegation were permissible, it would still be unconstitutional for Congress to pass laws punishing individuals who violated these regulations without due process and trial by jury - both rights guaranteed by Amendment VII of the Bill of Rights. In conclusion, Justice Johnson concluded that while there may have been good intentions behind this legislation passed by Congress, it could not stand up against constitutional scrutiny and thus must be struck down as invalid and voidable in order for justice to prevail in this case.