'Freewill Offering' in the Bible

Thou mayest not eat within thy gates the tithe of thy corn, of thy wine and of thy oil, either the firstborn of thine oxen or of thy sheep, neither any of thy vows which thou vowest, nor thy freewill offerings or heave offerings of thine hands:

and keep the feast of weeks unto the LORD thy God, that thou give a freewill offering of thine hand unto the LORD thy God according as the LORD thy God hath blessed thee.

but that which is once gone out of thy lips, thou must keep and do, according as thou hast vowed unto the LORD thy God a freewill offering which thou hast spoken with thy mouth.