Am I obligated to repay my wife the price of her gold which she sold to expedite our son's operation, knowing that I did not agree to its sale, and if so, should I return it as gold or as a cash amount?

Since the wife voluntarily bore the expenses of this procedure, the husband is not obliged to repay her anything, except as a kindness from him. In case of dispute, the judiciary will decide the matter based on the evidence.