"Дураки врываются" - попробуйте:
SELECT
c.cst_Name,
m.invcm_DocNo,
m.invcm_Date,
m.invcm_TotalAmount,
m.recm_DocNo,
m.recm_Date,
m.recm_TotalAmount
FROM dbo.SI_Customer c
LEFT JOIN (SELECT cst_Code,
invcm_DocNo,
invcm_Date,
invcm_TotalAmount,
CONVERT(varchar(10), null) recm_DocNo,
CONVERT(datetime, null) recm_Date,
CONVERT(decimal(25,8), null) recm_TotalAmount
from dbo.SI_InvoiceMaster
where cst_Code = '001813'
union all
SELECT cst_Code,
CONVERT(varchar(10), null) invcm_DocNo,
CONVERT(datetime, null) invcm_Date,
CONVERT(decimal(25,8), null) invcm_TotalAmount,
recm_DocNo,
recm_Date,
recm_TotalAmount
from dbo.SI_ReceiptMaster
where cst_Code = '001813') m
ON c.cst_Code = m.cst_Code
WHERE c.cst_Code = '001813'