ReceiverGroup       PayerGroup            Value
GroupA              GroupA              1000
GroupA              GroupC              3000
GroupA              GroupC              6000
GroupB              GroupD              2500
GroupB              GroupA              5000
GroupB              GroupC              2500
GroupE                  GroupA              1500
GroupE                  GroupB              3500

SELECT DISTINCT Group, Received, Paid
FROM (
      SELECT a.ReceiverGroup AS Group, SUM(a.Value) as Received, 0 as Paid
      FROM Test a
      Group by a.ReceiverGroup
      UNION ALL
      SELECT b.PayerGroup as Group, 0 as Received, SUM(b.Value) as Paid
      FROM Test b
      Group by b.PayerGroup
) AS c

Group              Received        Paid
GroupA              0           7500
GroupA              10000         0
GroupB              0           3500
GroupB              10000        0
GroupC              0         11500
GroupD              0         2500
GroupE                  5000          0

Group                 Received              Paid
GroupA              10000                 7500
GroupB              10000                 3500
GroupC              0                        11500
GroupD              0                        2500
GroupE                 5000                    0 

SELECT Group, SUM(Received), SUM(Paid)
FROM (
      SELECT a.ReceiverGroup AS Group, SUM(a.Value) as Received, 0 as Paid
      FROM Test a
      Group by a.ReceiverGroup
      UNION ALL
      SELECT b.PayerGroup as Group, 0 as Received, SUM(b.Value) as Paid
      FROM Test b
      Group by b.PayerGroup
) AS c
GROUP BY Group