Answer: You can't.

What you can do is to give a discount specifically for this client for one particular SKU (or any client and couple SKUs if desired). 


Discounts are covered in this article


For example:
Let's say you want to set price tier 1 for a customer but for one particular item you give him a discount $2 because he found a better price. 
You can set a discount rule "$2 off" (or you can use %) and specify the discount for the product as per example:






Here I set a discount rule for all customers with the tag, but you can also specify individual discounts for customers.


Note: The price for this item is $10




However when I click on the Apply discounts button the price changes to $8