'Voluntary offering' in the Bible

And, whosoever is left, of all the places where he doth sojourn, let the men of his place uphold him, with silver and with gold, and with goods and with beasts, - along with a voluntary offering for the house of God, which is in Jerusalem.

and, afterwards, the continual ascending-sacrifice, and, on the new moons, and on all the appointed seasons of Yahweh, the hallowed things, - also for every one that volunteered a voluntary offering unto Yahweh.