'Free will offering' in the Bible

And thou shalt keep the feast of weeks unto Jehovah thy God with a tribute of a freewill-offering of thy hand, which thou shalt give, according as Jehovah thy God blesseth thee:

That which is gone out of thy lips thou shalt observe and do; according as thou hast vowed unto Jehovah thy God, a freewill-offering, which thou hast promised with thy mouth.