Answer:
(Fatwa: 1038/927=/1429)
If you are sure or nearly sure that it is haram money then you should return it to him, otherwise you are not responsible to investigate the matter, you are responsible only to distribute it among the poor correctly:
و حمل فعل المسلم علی الصحۃ والحل واجب ما امکن الا ان تقوم البینۃ (المبسوط للسرخسی: 17/735)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
