In an action to recover commissions for a sale of land, where the evidence shows that the intended purchaser was able, ready and willing to perform, but defendant claims that plaintiff prevented the fulfillment of the agreement by defendant within the time specified in the contract, the question whether the negligence of plaintiff was the cause of defendant's failure to perform is for the jury. Stauff v. Bingenheimer, 94 Minn. 309, 102 N. W. 694. See also Sec. 899.