Actually, from the legal perspective, the employee must be paid full salary for notice period he is willing to serve, unless provided otherwise by the appointment letter (many appointment letters are very well crafted giving the company right to relieve him immediately).
So it you need to have a look at your appointment letter for that.
However, from a humanity point of view, and considering future relationship (you do not know if he will join your client, for example) it makes sense not to save part of his salary. Its a call you need to make on that part also