How we change what others think, feel, believe and do
Tell the customer that you only have limited authority to give discounts and that you will have to ask your manager to give more. You can at this point ask if the customer wants you to do this.
Then go to speak with the manager. Come back and say you have managed to persuade the manager to give an extra discount.
I'm sorry that's all the discount I'm allowed to give. But
we're a bit below target and I'm sure if I ask my manager she might shave a
little more off for you. Is that ok?
The basic principle here is that in approaching their manager, the sales person is setting up an exchange: they have put themself out for you, so you are now obliged to do something for them -- like buy the product. This is amplified as the manager can be a fear-inspiring figure of authority, thus making the salesperson a courageous warrior, fighting on behalf of the customer.
Having to ask the manager also puts a break in discounting and some people will pay the given price rather than add the hassle of the manager's intervention.
There are several variants of what happens when you go to speak with the manager, including whether the manager is visible and whether the manager comes to speak with the customer. A simple but effective method is that the customer can see you arguing with the manager and clearly taking some heat on their behalf. You then return with the final offer.
And the big