'Voluntary offering' in the Bible

Then you are to celebrate the Festival of Weeks before the Lord your God with the voluntary offering that you will bring, in proportion to how he has blessed you.

Then I said to them, "You are holy to the Lord, just as these vessels are holy. The silver and the gold are a voluntary offering to the Lord, the God of your fathers.

When the prince provides a freewill offering, a burnt offering, or peace offerings as a voluntary offering to the Lord, the gate facing east will be opened for him, and he will provide his burnt offering and his peace offerings just as he did on the Sabbath. Then he will go out, and the gate will be closed after he goes out.