بسم الله الرحمن الرحيم
(Fatwa: 580/575/N=06/1437)
It is unlawful and haram to deposit a large amount in the bank or Post Office as fixed deposit and receive monthly or annual interest on it. It Is not lawful in Islam. If the person mentioned in the question cannot do the business himself he can do it with partnership of some relatives such as his bhanja or bhatija etc or buy some shop or house and give it on rent as it is a lawful income source on monthly basis.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband