A is in urgent need of money for marriage of his daughter therefore agrees to sale his house to B at a price lower than the market price. Is this agreement valid?

The important element for converting an agreement into a contract is the presence of Consideration, however, it is not essential that the consideration should be adequate. Therefore, consideration may or may not be adequate but it should be of some value in the eyes of the law. Also, an agreement to which the consent of the promisor is freely given is not void merely on the basis that the consideration is inadequate; but the inadequacy of the consideration may be taken into account by the Court in determining the question whether the consent of the promisor was freely given or not.