The district court shall establish a date upon which all petitions for the approval of deeds to lands inherited by full-blood Indians shall be heard which date shall be at least twice per month. At a hearing the court shall take the testimony of disinterested parties to establish the value of the lands to be conveyed, and may in its discretion call in any United States probate attorney as counsel for grantor. The attendance of the grantor or any other person to testify in such matters may be required in like manner as if his testimony were to be heard in a civil action. The court may decline to approve any deed when in his judgment the price is not commensurate with his fair judgment and the grantor is not an heir of deceased. Provided, the district court may authorize the sale and conveyance of any such land for cash or one-fourth ( ¼ ) cash and the balance in yearly payments of one-fourth ( ¼ ) each, with interest on such deferred payments at the rate of not less than six percent (6%) per annum; provided further, that in all sales upon deferred payments such payments shall be secured by first mortgage upon the lands conveyed.