'Free will' in the Bible

The firstlings of all the first fruits, and all free will offerings shall be the priest's. Ye shall give unto the priest also the firstlings of your dough, that God may prosper the residue.

All the people of the land shall give this heave offering with a free will.

“When the prince makes a freewill offering, whether a burnt offering or a fellowship offering as a freewill offering to the Lord, the gate that faces east must be opened for him. He is to offer his burnt offering or fellowship offering just as he does on the Sabbath day. Then he will go out, and the gate must be closed after he leaves.