In 1773 Warren Hastings, then Governor General for the East India Company in India, took over production of opium. Trade with China had become one of the Company’s most significant activities and opium from India was its most wanted product in China, so this was a way to boost the Company’s revenue at the expence of the private British merchants who then ran the trade. 
Production was mainly at Patna, upstream on  the Ganges from Calcutta and downstream from Benares. At Patna, the opium juice was dried in the sun then moulded into cakes. Hastings sold the monopoly first to a Mr Mackenzie, who sold it on to Hastings’s servant Sulivan who quickly made a large sum and sold it on to John Benn, the future husband of Margaret Fowke, sister of Francis who built Boughrood Castle. John Benn sold it on almost immediately for another large profit, all this in 1773. 
John Benn was at that time secretary to Mr Markham, son of the Archbishop of York and  then the Company’s Resident at Benares. In 1774, the following year, Markham was replaced by Francis Fowke and Benn continued at Benares working for his new boss, thus his relationship with Francis’s sister Margaret when she came to stay with her brother a little later. 
It seems likely that Benn was at that point, 1774, wealthier than Francis. It also helps to explain why Benn, when he returned to England in 1786 on the same ship with Margaret and Francis, was still as rich or slightly richer, than Francis. It was on the ship that he became engaged to Margaret – they were married in 1787 in London.