Singning in the sales invoice by another person (staffs)

Dear Sir / Madam,

       We are the Pvt Ltd company and Having 3 directors (Mr A, B & C). One of the Director (Mr B) is Signing sales invoices and other documents. Both of others not in company at anytime. Some times Mr B is not available in the Factory/Office. At the time the sales staff signed the Invoices. As per my view It's not correct. Then 

My Questions are ...

What is the exact procedure for above...?

How can we deal this one...?

Please guide to me for the above matter...