Charles Spurgeon Commentary 1 Chronicles 21:20-23

"And Ornan turned back, and saw the angel; and his four sons that were with him hid themselves. Now Ornan was threshing wheat. And as David came to Ornan, Ornan looked and saw David, and went out of the threshing-floor, and bowed himself to David with his face to the ground. Then David said to Ornan, Give me the place of this threshing-floor, that I may build thereon an altar unto Jehovah: for the full price shalt thou give it me, that the plague may be stayed from the people. And Ornan said unto David, Take it to thee, and let my lord the king do that which is good in his eyes: lo, I give [thee] the oxen for burnt-offerings, and the threshing instruments for wood, and the wheat for the meal-offering; I give it all." — 1 Chronicles 21:20-23 (ASV)

Now Ornan was threshing wheat. And as David came to Ornan, Ornan looked and saw David, and went out of the threshingfloor, and bowed himself to David with his face to the ground. Then David said to Ornan, Grant me the place of this threshingfloor, that I may build an altar therein unto the LORD: thou shalt grant it me for the full price: that the plague may be stayed from the people. And Ornan said unto David, Take it to thee, and let my lord the king do that which is good in his eyes: Lo, I give thee the oxen also for burnt offerings, and the threshing instruments for wood, and the wheat for the meat offering; I give it all.

And as we are told in the other narrative, as a king gives to a king, so did Araunah to David. Probably he had been a king, and David had dispossessed him in his conquest of Jebus, but now he proves that he had a royal heart, and he offers to give all to King David.