Offerings: Free Will: Obligatory when Signified in a Vow
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Deuteronomy 16:10
And you shall celebrate the Feast of Weeks to the LORD your God with a freewill offering that you give in proportion to how the LORD your God has blessed you,
Deuteronomy 23:23
Be careful to follow through on what comes from your lips, because you have freely vowed to the LORD your God with your own mouth.