A contract by which one of the contracting parties called the “seller” enters into an obligation to the other to cause him to have freely by a title of proprietor a thing for the price of a certain sum of money which the other contracting party called the “buyer” on his part obliges himself to pay. Poth. Cont.; Civ. Code La. 1000 art. 21.10; White v. Treat (C. C.) 100 Fed. 201; Sawmill Co. v. O