2 Cor 8:12

8:12 As the widow’s offering in Mark 12:41–44 teaches, willingness to give generously is pleasing to God, even though the gift may have to be small because the giver is poor.

not according to what he does not have. This is a warning against giving or promising to give an amount that you really do not have, hoping that God will repay it. Doing this forces a test on God (Luke 4:12). People should give as God causes them to prosper (1 Cor. 16:2). Even so, the more common offense is failing to give immediately and generously when God brings additional income.