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.
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.