Hi S. C Kanti
With reference to your post, what I can suggest is you may issue a circular or letter to all the dealers that the concerned Manager has resigned from the job and moved on, draft the letter in such a positive way to put across the message that manager is not working with the company to avoid any miscommunication, also in the same mail, mention the contact details of the replacement person on the vacated position or a temproray contact person untill further information.
Something like
Dear Dealers
Please be informed, that Mr XYZ has resigned from the post of Manager, however we shall continue to nurture our business relationship as usual. Please note contact details of Mr ABC who shall take over or in-charge of your account henceforth / hereafter. Please feel free to discuss your concerns related to business transactions. Looking forward to continue our business relationship.
Mr. ABC - Job title
E mail :
Mobile :
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I suggest don't mention anything about termination of the employee, even if the employee is terminated let that be inside boardroom walls.
With regards to informing field staff, you may convey to them by a normal memo or mailers sent to concerned field staffers regarding Mr. XYZ not working with the company anymore and inform contact details of the personnel taken charge.
Hope this helps