'To raise a present sum'. How much was this sum? Why was it required urgently

Actually the sum of money was "3000 ducats or dollars".

It was needed urgently by Bassanio because he had to go Belmont as fast as he can. The reason for urgently going to Belmont for Bassanio was he loved Cato' s daughter "PORTIA". Besides having great qualities and virtues she also was much beautiful. So for marrying her Bassanio was needed to go at Belmont.