Akshay,
As Joao hss explained, after 10 qty was sold a bill was created for 100 USD.
Next, customer returns 1 qty, worth 10 USD
The bill of the sale can not be changed.
A new bill (credit memo) will be created, which will lessen the payment to be done by the customer to 90 USD.
As one can not change the bill of the sale, another bill is to created for crediting the customer.
If you create another bill of 90 USD , the payment by customer becomes 100 + 90 = 190 USD, which is incorrect.