Dear Aditenair
First of all I on behalf of all the members of CiteHR Welcoming you to join in this Forum.
Please go through
https://www.citehr.com/118961-welcom...ers-group.html thread for gaining the adequate subject knowledge and to know the procedures to be followed in this forum.
You have issued the Offer letter to a candidate. Have you obtained a signature on the second copy stating that he is accepting the offer and join on some specific date.
In that acceptance sign column itself you can add one more clause as mentioned by Mr. Ashok Jangra.
Otherwise you cannot take any legal action or any penalty for non compliance.
Wish you happy learnings.