One possible workaround in Odoo 11 is to create a portal user in Odoo and then link that to an employee.  But in later versions it's only possible to select an internal user in the group Sales / User: Own Documents Only).

Hi,

The field sales person is a many2one relation to res.users(Users table). So if you want to select employees for field sales person, You will have to create a custom module for that.

I have the same issue and cannot find a solution other than creating users with fees. 

You can grant an employee a portal user access through: contacts, select employee, action, portal access management.

After that you will have to go to manage users and grant the same employee access to POS module as a user. 

But I am not sure if this will not get you charged anyways since you giving the POS access to an employee. Also I heard that using portal user access to an employee is illegal.