I read that Maharaj caused Kashiram to face financial difficulties. Why would he do that to a devotee?

The text from Chapter 0 explains this as a divine 'Leela' to help Kashiram's spiritual progress. Kashiram had developed pride in his ability to give Dakshina, feeling bad if Maharaj didn't accept it, which is considered an obstacle on the spiritual path. To remove this pride, Maharaj began demanding more and more Dakshina, to the point where Kashiram had to borrow money and eventually ran out. This difficult experience forced Kashiram to realize that he did not truly have the 'power to give.' As soon as this realization dawned and his pride was removed, his financial condition began to improve again.