Numbers 30:2
If a man vow a vow unto the LORD, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth. If a man vow a vow unto the LORD, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth.
A vow to the Lord binds the maker not to break his word but to do according to all that has proceeded out of his mouth. A pledge to give for God's cause, including a specific amount, is such a vow made to God and not merely a promise to people. It must be treated as sacred and faithfully fulfilled rather than as casual or revocable.
This includes pledges toward building a house of worship, whether the promised words are spoken aloud or written, which are sacred and should not be broken. Such giving vows may be made in response to the Spirit's work when people resolve to deny self for God's cause. They are made before heavenly witnesses, and when a person moved by the Holy Spirit promises a specific amount, that promised portion is consecrated so it is no longer the giver's to withhold.
Failing to fulfill the vow is a serious offense. Withholding part of what was promised while pretending to fulfill it is fraud against God, and promises to God are binding not less than agreements made with people. The tithe is the tenth that God claims, while giving more than the tithe is voluntary unless a person has bound the soul by vow to give it.