بسم الله الرحمن الرحيم
It should be our belief that we will get whatsoever is destined for us in each case. We must not fall prey of superstition. If our income is decreasing and there is no increase in it then it is wrong to think that someone has done black magic. Decrease in livelihood may be due to our misdeeds. Without any proof and evidence accusing someone for magic or developing misconception about someone is sin and unlawful.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.