can you please try this query..
SELECT
Prepaid.RedeemerID,
OneTimePin.WithdrawerID,
SUM(ISNULL(Prepaid.Amount,0)) AS PrepaidAmountSum,
SUM(ISNULL(OneTimePin.Amount,0)) AS OneTimePinSum,
SUM(ISNULL(Prepaid.Amount,0))- SUM(ISNULL(OneTimePin.Amount,0)) AS 'Difference'
FROM Prepaid
INNER JOIN OnetimePin ON Prepaid.RedeemerID = OneTimePin.WithdrawerID
WHERE Prepaid.IsActive = 1 AND OneTimePin.IsActive = 1 AND Prepaid.IsRedeemed = 1 AND OneTimePin.IsPaid = 1
GROUP BY Prepaid.RedeemerID,OneTimePin.WithdrawerID
ORDER BY 'Difference', Prepaid.RedeemerID, OneTimePin.WithdrawerID