Selz v. Unna was a United States Supreme Court case that dealt with the issue of whether a contract between two parties was valid. The case involved a dispute between a German immigrant, Selz, and a German-born American citizen, Unna. Selz had agreed to pay Unna a certain amount of money for the use of a piece of land. Unna then sold the land to another party without Selz's knowledge or consent. Selz sued Unna for breach of contract, arguing that the contract was valid and binding. The Supreme Court held that the contract was valid and binding, and that Unna had breached it by selling the land without Selz's knowledge or consent. The Court noted that the contract was made in good faith and that both parties had intended to be bound by it. The Court also noted that the contract was not voidable due to any fraud or misrepresentation on the part of either party. The Court's decision in Selz v. Unna established that contracts between two parties are valid and binding, even if one of the parties is an immigrant. This decision has been cited in numerous subsequent cases involving contracts between immigrants and American citizens.

In Selz v. Unna, the Supreme Court was asked to decide whether a contract between two parties could be enforced when it had been made without consideration. The majority opinion held that no consideration was necessary for an enforceable contract; however, Justice Field dissented from this decision and argued that contracts must have some form of consideration in order to be valid and binding on both parties. He reasoned that if there is no exchange of something valuable or beneficial between the contracting parties then there is nothing preventing either party from simply walking away from their agreement with impunity. Furthermore, he believed that allowing contracts without any form of consideration would lead to fraud and abuse as unscrupulous individuals could take advantage of unsuspecting victims by making promises they never intended to keep. Ultimately, Justice Field concluded his dissent by stating his belief that all agreements should require some sort of mutual benefit in order for them to be legally binding upon both sides involved in the transaction.