Export suplus of india were used to make payments for the expenses incurred by office set up by the colonial government in Britain.
1. confusion is how they used the money to pay in british currency
2. and what were the expenses that british office could have incurred because of which they had to pay for.??
As per the Coinage Act of 1835, throughout India there was an uniform coinage. It was basically minted by the British in India. There was no Indian rupee untill the WW1 (The First World War), when the shortage of British silver coins forced the introduction of Indian Rupee notes. Thus, all these basically means that during the colonial rule it was British currency that circulated throughout India. Thus, the export trade surplus of India's foreign trade was in British currency.
In response to the second part of your question, British used the export surplus (from India's trade) in order to finance their own expenses. Basically, these expenses were administrative expenses (to keep their rule going in India), war expenses (to expand their colonies besides India), infrastructural development in colonies (to strengthen their grip over the colonies) and lastly to finance the Britain's economic development.