A sale is a contract whereby the parties agree that one of them shall transfer to the other his rights over property and the latter shall pay a price for it.
The contract of sale is completed when the parties have mutually agreed on the object and the price.

Although the price is not fixed concretely by the agreement, if it may be decided from the circumstances, it is deemed to be fixed.
If it is agreed that the price shall be fixed according to the market price, it is deemed to be fixed according to such market price at the place and time of performance unless otherwise provided by contract.

The provisions under the present title shall apply mutatis mutandis to such nongratuitous contracts other than those of sale, unless the nature of the contract does not permit.