'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:

The produce of thy lips thou dost keep, and hast done it, as thou hast vowed to Jehovah thy God; a free-will-offering, which thou hast spoken with thy mouth.