It sounds like you have a sales manager who is far more sales than manager. He still likes to get out there and mix it up with the customers, without realizing that all he is doing is getting everybody mixed up.
It makes a good deal of sense for a sales manager to go on some sales calls with the salespeople, and even to get more actively involved in the process if the salespeople are struggling. But, it makes no sense at all for the manager to try to take over the sales presentation of a successful salesperson in a successful territory.
Most successful salespeople are not particularly shy, and this is no time for such behavior. You should stay in your sales mode and let your sales manager know you appreciate what he is trying to do, but clearly let him see how you feel about this. Be sure to let him see how his actions are costing him and the company money.
A sales manager should coach, guide, and support the sales team. There can be cases where the manager gets directly involved in the sales process, typically to help or to demonstrate various selling techniques. But, no salesperson is going to buy the notion of a sales manager taking over the whole presentation.
| 
