see the last price that the customer paid for the item in the little drop down area when the part number comes up

The boss wants to be able to see the last price that the customer paid for the item in the little drop down area when the part number comes up. For example, I am writing a sales receipt and I type in “Item#1234” and the drop-down menu shows the item numbers starting with this number, including the price that this was last sold to this customer before.