Make sure the letter includes language stating that this is not a contract and reserving your right to make changes as needed. (This can work to the employee's benefit too; if the offer letter is a considered a contract s/he would never be entitled to a raise, or would not be entitled to benefits if you sometime in future were in a position to offer them.) You should probably have it reviewed by your corporate counsel to make sure you do not inadvertantly create a contractual obligation.

