Thank you for your assistance! After googling a few pages I came across this query. I adjusted it to comply to my needs.
Herewith query if you guys are interested:
SELECT
T0.DocNum AS 'Invoice Nr',
T0.NumAtCard AS 'Customer Ref Nr',
T1.SlpName AS 'Salesperson',
--T0.SlpCode AS 'SP Code',
T0.DocDate AS 'Posting Date',
T0.CardCode AS 'Customer Code',
T0.CardName AS 'CustomerName',
'Credit Memos' AS 'Doc Type',
((T0.DocTotal) * -1) AS 'Total Docs',
(((T0.DocTotal - T0.VatSum) - T0.TotalExpns) * -1) AS 'Product Sales',
((T0.GrosProfit) * -1) AS 'Gross Profit',
CASE
WHEN (((T0.DocTotal - T0.VatSum) - T0.TotalExpns) * -1) = 0.00 THEN 0.00
ELSE (((T0.GrosProfit) * -1)/(((T0.DocTotal - T0.VatSum) - T0.TotalExpns) * -1)) * 100
END AS 'Gross Profit %',
((T0.PaidToDate) * -1) AS 'Paid To Date',
((T0.DocTotal - T0.PaidToDate) *-1) AS 'Open on Docs'
FROM ORIN T0
INNER JOIN OSLP T1
ON T0.SlpCode = T1.SlpCode
WHERE
(T1.SlpName LIKE '%%[%0]%%' OR '[%0]' = ' ')
AND T0.TaxDate >= '[%1]'
AND T0.TaxDate <= '[%2]'
UNION
SELECT
T0.DocNum AS 'Invoice Nr',
T0.NumAtCard AS 'Customer Ref Nr',
T1.SlpName AS 'Salesperson',
--T0.SlpCode AS 'SP Code',
T0.DocDate AS 'Posting Date',
T0.CardCode AS 'Customer Code',
T0.CardName AS 'CustomerName',
'Invoices' AS 'Doc Type',
(T0.DocTotal) AS 'Total Docs',
((T0.DocTotal - T0.VatSum) - T0.TotalExpns) AS 'Product Sales',
(T0.GrosProfit) AS 'Gross Profit',
CASE
WHEN (((T0.DocTotal - T0.VatSum) - T0.TotalExpns)) = 0.00 THEN 0.00
ELSE ((T0.GrosProfit)/((T0.DocTotal - T0.VatSum) - T0.TotalExpns)) * 100
END AS 'Gross Profit %',
(T0.PaidToDate) AS 'Paid To Date',
(T0.DocTotal - T0.PaidToDate) AS 'Open on Docs'
FROM OINV T0
INNER JOIN OSLP T1
ON T0.SlpCode = T1.SlpCode
WHERE
(T1.SlpName LIKE '%%[%0]%%' OR '[%0]' = ' ')
AND T0.TaxDate >= '[%1]'
AND T0.TaxDate <= '[%2]'
Thanks again for the replies!