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