Need an SQL joining lesson

Not the greatest at SQL but have a project where I think it's a perfect fit
so I want to figure this out. It's ultimately going into an RPG program
(as an INSERT statement), but the question at the moment is the SQL, not
RPG. That's why what you see below is a SELECT with stuff hardcoded. I
just want the right numbers first.

Some customers are going to start getting rebates. The rebate will a
percent of (sales - credits) by item category for the month. I want to
insert into a file the following 5 fields:

customer number
YYYYMM
category
total sales for the month
total credits for the month

There is a file of customers and their rebate percents, the sales file
(with the category field in it), and a credits file (with the category
field in it). There could be both sales and credits for a
customer/category, only sales for a customer/category, or only credits for
a customer/category. In all 3 cases I want 1 record per customer/category.

CUSRBTMST - rebates percents file
SLSALL - sales file
CRDMEDTL - credits file

This is the SQL I have now:

SELECT m.cusnr,
201802 AS YYYYMM,
COALESCE(s.catcd1, c.catcd1, ' ') AS category,
COALESCE(SUM(s.itnsa), 0) AS Sales,
COALESCE(SUM(c.cdtam), 0) AS Credits
FROM DBMSTF.CUSRBTMST m
JOIN SLSHST.SLSALL s ON m.cusnr = s.cusnr
JOIN DBMSTF.CRDMEDTL c ON m.cusnr = c.cusnr
WHERE s.INVDT BETWEEN '2018-02-01' AND '2018-02-28' AND
c.DTWRTISO BETWEEN '2018-02-01' AND '2018-02-28'
GROUP BY m.CUSNR, s.catcd1, c.catcd1
ORDER BY m.CUSNR, s.catcd1, c.catcd1

That's a copy/paste from ACS run SQL scripts that I formatted a bit further.

The results I get have no resemblance to the actual sales and credits. :)

I'm joining the rebate percent file to both the sales and credits file.
That might be the problem, I don't know. Are 2 SELECTS required?

Thanks for any pointers.