Read history. He gave a promise to his army that after beating muslims because it will take long time. All will come back home. Thats why his army was unstopable. He promissed that they will return with what they gather. Another thing is that sometimes he was giving all the captured treasures to his men. India was not his point of war. Budism and all your story has nothing to do with that.
To keep power you have to accomplete your promisses given to your men.