Quick answer
The chapter's answer to verse 8. What looked like a precious stone to its owner is named here as the corruption of justice.
Read both lines
The NKJV footnotes "behind the back" as under cover, literally from the bosom, meaning the payment comes out of a fold in a garment where nobody sees it. That detail is the tell: the transaction requires concealment because everyone involved knows what it is. Read against verse 8, the chapter has now shown the same act twice, once as the participant experiences it and once as it actually is. And the purpose clause is what ties it to verse 15. This is how a judge comes to call a guilty man righteous.
Related: Proverbs 17:8 · Proverbs 17:15 · Exodus 23:8 · Isaiah 5:23
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.