The defendant having assented to the terms of the written agreement to exchange, the agreement and the deed of conveyance were competent evidence of the sale and the consideration thereof. Hewitt v. Brown, 21 Minn. 163; Folinsbee v. Sawyer, 157 N. Y. 196, 51 N. E. 994; Levy v. Coogan, 9 N. Y. S. 534, 16 Daly 137; Cannon v. Castleman, 24 Ind. App. 188, 55 N. E. 111. Compare Sec. 159.