Hi Suresh,
Though you have the sales order shown as full value, you can always edit to the vaue as required - Please check.
However, once your second sales order is created for 20,000 then the Sales Contract will have 60,000 as its remaining value (Sales Order 1 = 20,000 + SAles Order 2 = 20,000 -> = 40,0000). Once sales orders are created for 1,00,000 then you cannot create further sales orders.
Regards