Farni v. Tesson was a case heard by the United States Supreme Court in 1861. The dispute arose when John Farni, an Italian immigrant living in New Orleans, Louisiana, sold his house to Joseph Tesson for $2,000 and received payment from him with two promissory notes of equal value. When Farni attempted to collect on the notes he discovered that they had been altered without his knowledge or consent so as to increase their amount due by several hundred dollars each. He then sued Tesson for fraudulently altering the documents and sought damages from him for breach of contract. The Supreme Court held that while it is true that parties may alter contracts after they have been signed if both parties agree upon such alteration; however this must be done openly and fairly with full disclosure made between them regarding any changes being made before either party signs off on them again. In this case there was no evidence presented which showed that any such agreement existed between Farni and Tesson thus making it impossible for the court to find in favor of either party's claim against one another

In the case of Farni vs. Tesson, the Supreme Court was asked to decide whether a contract between two parties could be enforced when it had been made in violation of an existing state law. The majority opinion held that such contracts were not enforceable and thus dismissed the appeal. However, Justice Grier dissented from this decision on the grounds that while he agreed with his colleagues that state laws should generally be respected, there were certain instances where upholding a contract would serve justice better than disregarding it. He argued that if both parties had entered into an agreement in good faith and without knowledge of any applicable law prohibiting their actions then they should have their rights upheld by enforcing said contract as long as no third party was harmed or prejudiced by its enforcement.