Hide button from sales manager and only sales person can view

i have created a button 'Quotation to Approve' in a sale order form so that every time salesperson created a quotation he/she need to click that button to get approval from manager. however when sales manager want to create quotation, he/she also can view that button. i want to hide that button from sales manager and only sales person can view it. i tried to use groups=base.group_sale_salesman but still manager can view it. any ideas? i believe someone already done this before. hopefully i can get the solution here. thanks in advance.