Question

what caused huge loss of levenue in bengal

Answer 1

The conflict between Bengal Nawabs and the East India Company started when the nawabs refused to grant the company concessions and demanded large tributes for the company's right to trade.The nawabs also denied the the company any right to mint coins and stopped it from extending its fortifications.

Answer 2

The nawab of Bengal at that time was Sirajuddaulah. The farman given was used for personal purpose by the  britishers . This gave a huge revenue loss to the Bengal .