In Islam, earning money from money is lawful when one has to use time, wit and labour with it. It is not lawful to give someone money and take fixed profit from him monthly. As sometimes there is less profit in business and some time more while sometime it is equal, and some time there is loss and damage as well. The situation is not similar in each month or year. The businessmen know it very well. Thus it is not lawful as per the Shariah to take fixed profit, it shall become interest i.e. it shall be like exchanging money for money, Shariah does not approve it.
Allah knows Best!
Darul Uloom Deoband