This is not permitted but comes under Riba a-Fadl. However, to escape from the great sin and unlawfulness of Riba (Interest), if on the side that has lesser gold some thing else was added, even if it be some money, then it would be permitted. For example, if 500 grams of gold is being exchanged for 700 grams, if with the 500 grams £5 (for example) is added, then it will be said that the 500 is being opposed by 500 and the 200 is for the £5.