It appears that Naaman was acknowledging the God of Heaven, Israel’s God as the one true God.
It would have been insulting to God for the prophet to receive anything from Naaman when God is the proprietor and the progenitor of wealth especially on His turf and in His land.
If God was able to cleanse a man of an incurable skin condition why would God need that person’s gold, or slaves or cattle?
Notice Naaman asks for dirt to take back home with him.
This was God’s land. And this is what God said about His land
“You shall remember the Lord your God, for it is he who gives you power to get wealth, that he may confirm his covenant that he swore to your fathers, as it is this day.”
And if you read the verses earlier the categories that Elisha mentioned to Gehazi are listed out as belonging to the promised land.
“"Take care lest you forget the Lord your God by not keeping his commandments and his rules and his statutes, which I command you today, lest, when you have eaten and are full and have built good houses and live in them, and when your herds and flocks multiply and your silver and gold is multiplied and all that you have is multiplied, then your heart be lifted up, and you forget the Lord your God, who brought you out of the land of Egypt, out of the house of slavery,”
Elisha hearing from God refused out of obedience to God anything Naaman offered because after all he was entitled to payment as a prophet. I speculate that God seeing a true turning of Naaman to Himself did not want Naaman to say that he contributed toward his conversion, especially not with material and perishable things