"€500,000 of a private loan" would indicate that the total, overall loan was larger than €500,000, and that "he" got a portion of the total loan. The portion "he" got was €500,000. We don't know the total amount of the loan; it could have been as little as €500,001 or, I suppose, even €500,000.01.
That's not what the original phrase said. Why do you want to change it?