What is the ruling regarding a car that was sold with an agreement to pay one-third of the amount immediately and postpone the remainder for ten days or return it, and then the buyer used it for five months, changed its engine, and did not pay the remaining amount?

The seller's condition of having the option to return the car or other item if the buyer fails to pay the price due to inability or procrastination is valid. The buyer must pay the remaining price. If he does not, the seller has the right to annul the sale and reclaim what he had delivered. However, if the buyer has already disposed of the sold item, he is only obligated to pay the remaining price, and the sale is not annulled. If he refuses, the matter should be taken to court to compel him.