Deuteronomy 23:21
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. 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.
A vow or pledge made to God must be paid without delay. God will surely require its fulfillment of the one who made it. To slack in paying what was vowed is sin in the one who vowed it. No one is required to make a vow, and to refrain from vowing is not sin. But once anything is promised to God, including a freewill offering, it becomes binding and must be kept and performed.
Pledges made to build a house of worship are vows to God and not merely promises to people. They are solemnly witnessed and recorded, and those who make them must keep them. Pledges given to support God’s work are as binding as a note. They are a solemn obligation and should be paid before the money is invested elsewhere.