Article 442

When the Option of Sales Unfulfilled in Part comes into force, the portion of the consideration which must be returned to the purchaser is calculated as follows That part of the object of sale which becomes the property of the purchaser will be valued alone; the seller will keep that portion of the consideration which bears to the whole consideration of the purchase the same ratio as the value of the portion mentioned above bears to the value of the whole thing sold in its entirety; and the seller must return the rest to the purchaser.