I was trying another way, and here what I got:
SELECT
ClientNo,
ClientID,
FullName,
SUM(InvoiceTotalDue) as InvoiceTotalDue,
sum(InvoiceTotalBalance) as InvoiceTotalBalance,
sum(ReturnChequeTotalDue) as ReturnChequeTotalDue,
sum(ReturnChequeTotalBalance) as ReturnChequeTotalBalance
FROM (SELECT
CT.ClientNo,
CT.ClientID,
CT.FullName, #IT.*,RCT.*
IT.TotalDue as InvoiceTotalDue,
IT.TotalBalance as InvoiceTotalBalance,
0.00 as ReturnChequeTotalDue,
0.00 as ReturnChequeTotalBalance
FROM Client_TBL as CT
JOIN Invoice_TBL as IT on (IT.ClientNo=CT.ClientNo)
UNION
SELECT
CT.ClientNo,
CT.ClientID,
CT.FullName, #IT.*,RCT.*
0.00 as InvoiceTotalDue,
0.00 as InvoiceTotalBalance,
IFNULL((RCT.TotalDue),0.00) as ReturnChequeTotalDue,
IFNULL((RCT.TotalBalance),0.00) as ReturnChequeTotalBalance
FROM Client_TBL as CT
JOIN ReturnCheque_TBL as RCT on (RCT.ClientNo=CT.ClientNo)) AS ResultSet
GROUP BY ClientNo;
If you want you can create a stored procedure to filter any information that you want.
create table ..
with some data at sqlfiddle.com/#!6. This will help us help you out