PST in Sask

I'm stuck with PST in Saskatchewan.  We are an automotive repair shop and were charged PST on parts that were resold to our customers and then charged PST again.  According to the Ministry on Finance, we are to deduct that amount paid on our monthly reporting.  I've done this, however, now my monthly report is showing as inaccurate, as though I didn't pay the full balance.  I had to submit this month manually as I couldn't get the numbers to work for the filing, and I still can't figure it out.  Please help!