Albert Barnes Commentary Psalms 54:6

"With a freewill-offering will I sacrifice unto thee: I will give thanks unto thy name, O Jehovah, for it is good." — Psalms 54:6 (ASV)

I will freely sacrifice unto thee - The Hebrew words rendered “freely” signify “willingness, voluntariness, spontaneousness.” The idea is that he would do it with a free or willing mind, without constraint or compulsion—voluntarily.

The reference is to a freewill or voluntary offering, as distinguished from one that was prescribed by law (Exodus 36:3; Leviticus 7:16; Leviticus 22:18; Numbers 15:3; Numbers 29:39).

The idea is that, as a result of the divine intervention for which he prayed, he would bring voluntary offerings to God in acknowledgment of his goodness and mercy.

I will praise thy name, O LORD - I will praise you .

For it is good - That is, God himself is benevolent, and David says that he would express his sense of God’s goodness by offering him praise.