Your letter of offer of employment must accompany a contract presented to an employee. Conditions of the employment contract and the date of offer expiring must be clear on the letter of employment. Anything connected to the possible future employment but outside of the individual employment contract must be included in your letter of offer and sent in writing with your signature.
The process of offering employment is formal, and the legal requirements are clear. Avoid potential misunderstandings and legal or personal grievances through using correct professional documents and processes from the start to the finish of your employee recruitment.