Contradiction with Proverbs 3:27
This verse instructs to not withhold good when it is in your power to act, as opposed to being thankful for diligent service already provided.
Proverbs 3:27: Withhold not good from them to whom it is due, when it is in the power of thine hand to do [it]. [them...: Heb. the owners thereof]
Contradiction with Luke 6:35
Encourages showing love and kindness without expectation of reciprocation, contrasting the gratitude expressed in 2 Corinthians 8:16 for someone's diligence.
Luke 6:35: But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and ye shall be the children of the Highest: for he is kind unto the unthankful and [to] the evil.