Contract Law.
In general, a contract can only exist if both parties are aware if its conditions.
Generally, a contract can only be enforced in a court of law if it is written, or some permanent record of it can be produced in court.
Ignorance of the law
Ignorantia juris non excusat or ignorantia legis neminem excusat (Latin for "ignorance of the law does not excuse" or "ignorance of the law excuses no one") is a legal principle holding that a person who is unaware of a law may not escape liability for violating that law merely because he or she was unaware of its content. In the United States, exceptions to this general rule are found in cases such as Lambert v. California (knowledge of city ordinances) and Cheek v. United States (willfulness requirement in U.S. federal tax crimes).
European law countries with a tradition of Roman law may also use the expression nemo censetur ignorare legem: nobody is thought to be ignorant of the law.
A contract is an agreement entered into by two parties or more with the intention
of creating a legal obligation, which may have elements in writing.
Contracts can be made orally.
The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific performance of the contract or an injunction. Both of these remedies award the party at loss the "benefit of the bargain" or expectation damages, which are greater than mere reliance damages, as in promissory estoppel.