Quick answer
He runs the seller down to get the price, then brags about the bargain on the walk home. The verse catches a small, universal dishonesty.
Read both lines
The Hebrew of the buyer's complaint is two words repeated, which the NKJV footnotes as literally evil, evil, and it is the sound of haggling: bad, bad, look at this. The comedy is in the timing. The verse follows him out of the market and lets us hear the second speech, which contradicts the first and which he would never give in front of the man he bought from. What the proverb has isolated is the moment a person's two accounts of the same event fail to match, and it does not moralize. It just places the two sentences next to each other and lets them do the work, four verses after a chapter that called rigged scales detestable.
Related: Proverbs 20:10 · Proverbs 20:23 · Proverbs 27:2
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.