Scott Hafemann, Second Corinthians, NIV Application Commentary, Zondervan, 2000, p. 381.
The New Testament does not teach a doctrine of tithing (i.e., the mandatory giving of 10 percent of one’s income). Nor does Paul define what constitutes giving generously. He does not even provide a target number or general guidelines. The only rule is to give freely and generously as an expression of our continuing trust in God’s grace (9:5-8). Paul simply assumes that believers will give all they can to meet as many needs as they can in order to glorify God as much as they can.
Scott Hafemann, Second Corinthians, NIV Application Commentary, Zondervan, 2000, p. 381.