Contradiction with Matthew 6:1
2 Corinthians 9:3 speaks of boasting about others' readiness to give, while Matthew 6:1 advises against public displays of righteousness to be seen by others.
Matthew 6:1: Take heed that ye do not your alms before men, to be seen of them: otherwise ye have no reward of your Father which is in heaven. [alms: or, righteousness] [of your: or, with your]
Contradiction with Proverbs 27:2
2 Corinthians 9:3 involves boasting about someone's zeal, whereas Proverbs 27:2 suggests letting another person's praise come from their own mouth, not your own.
Proverbs 27:2: Let another man praise thee, and not thine own mouth; a stranger, and not thine own lips.
Contradiction with James 4:16
2 Corinthians 9:3 may imply a sense of boasting about readiness, while James 4:16 warns that boasting and bragging are evil.
James 4:16: But now ye rejoice in your boastings: all such rejoicing is evil.