Could God have answered the prayer of a man who said to an inhabitant, "May God cut off your sustenance," and what is the solution if the reason for lack of success at work is this prayer?

You have not sinned by prohibiting your cousin from buying what is of no benefit to your grandfather. Rather, the sinner is he who wrongfully supplicated against you. Rely on Allah and entrust your affairs to Him, and strive in supplication. Know that what has afflicted you was not meant to miss you, and what has missed you was not meant to afflict you. And perhaps the delay in you finding a job contains good for you.