It seems clear to me that this person should borrow money to buy gold.  The person should make contracts to deliver the gold at the forward price in one year.  Then the person can hold the gold and deliver next year and make money.

To buy one ounce of gold now, the person needs $500.  If they borrow the entire sum and pay the interest rate you mention, their total cost for the ounce of gold will be $550.  You say that there is no storage cost and so their total cost for the gold is $550.

If you can buy for $550 and sell for $700, you should do it -- borrow as much as you can and buy as much gold as you can.