Unfortunately, merely signing a letter does not make it an enforceable contract. To be an enforceable conrtract, there must have an exchange of "consideration," or things of value, going to *both* parties. That is, you had to give your employer something in exchange for the raise. If you accepted a transfer to a different location, or began working much longer hours and/or took on new responsibilities, etc., in exchange for getting a raise in the future, those extra things you did may provide sufficient consideration to make the letter into a contract; in that case, you could potentially enforce it in court if necessary. But if you did not do anything new in order to get the raise, then there was no consideration and the letter is not enforceable.