A picture is worth a thousand words.

From The ESV Study Bible.
Abram shows both his bravery in rescuing Lot and his ongoing desire to honor God. Following his victory, Abram is led in worship by the priest/king Melchizedek (“king of righteousness”), who then receives Abram’s thanksgiving offering. Melchizedek is recognized in the New Testament as a type of Christ.
Hebrews 6:20-7:3 “where as a forerunner Jesus entered for us, having become a high priest forever after the order of Melchizedek. For this Melchizedek, king of Salem, priest of God Most High, who met Abraham returning from the slaughter of the kings and blessed him, to whom also Abraham divided a tenth part of all (being first, by interpretation, ‘king of righteousness’, and then also ‘king of Salem’, which means ‘king of peace’, without father, without mother, without genealogy, having neither beginning of days nor end of life, but made like the Son of God), remains a priest continually.”
In rejecting the king of Sodom’s offer of material goods Abram shows that he isn’t a bounty hunter in it for gain, or a vassal of a king. He is a servant of the “God Most High,” and he wanted any blessing he received to be unequivocally from the LORD.
Thought for the day: gifts to God through His servants are one way he has established for us to show thanks to him.
2 Corinthians 9:7 “Let each man give according as he has determined in his heart, not grudgingly or under compulsion, for God loves a cheerful giver.”