Legal Perspective on Notice Period Salary
Actually, from a legal perspective, the employee must be paid the full salary for the notice period he is willing to serve, unless otherwise specified in the appointment letter. Many appointment letters are well-crafted, giving the company the right to relieve him immediately. Therefore, you need to review your appointment letter for that clause.
Humanitarian Considerations
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 as well.