This Bible contradiction is from the Skeptic's Annotated Bible.
For fifty shekels of silver.
2 Samuel 24:24 View context
And the king said unto Araunah, Nay; but I will surely buy [it] of thee at a price: neither will I offer burnt offerings unto the LORD my God of that which doth cost me nothing. So David bought the threshingfloor and the oxen for fifty shekels of silver.
For six hundred shekels of gold.
1 Chronicles 21:25 View context
So David gave to Ornan for the place six hundred shekels of gold by weight.