Matthew Poole and continuators on Deuteronomy 23:21

21 When thou shalt vow a vow unto the LORD thy God, thou shalt not slack to pay it : for the LORD thy God will surely require it of thee: and it would be sin in thee .

To wit, if the matter of it be lawful and in thy own power. See Numb. 30. 2. Not slack, or delay, because delays may make thee both unable to pay it, and unwilling too, the sense of ones obligation growing every day weaker than other, &c.

(i. e.) It would be laid to thy charge as a sin and bring judgment upon thee.

Marginal references: Eccles. 5. 4.