Technically it is possible to do some workaround, but I suspect there is more to this question.

Can you give us an example of a couple of line items on an invoice and how you would like to calculate the GST?

For example,

Item 1    $100

Item 2    -$25

GST should calculate on only $75

If this is the example, then just put the sales tax code for GST on each line and the tax should calculate only on the $75.  So $5 on the sale on line one,

-$1.25 on the return on line two and the total tax would be $3.75 or 5% of $75.

Well... I just tried it again exactly the same way as the last time, and it worked.  

But, now I have another question.  

It is rounding my GST to $1.78, when it should be $1.77.  How do I adjust this?

Never mind, I got it.  

For some reason things don't work for me, then a few minutes later, it works.  Almost like the program needs a couple of minutes to update.

Thanks for the quick response!

Tax is calculated on each line item, rounded off to two decimals and then added up.  It is not calculated on the subtotal.

Double click either on the tax amount for one line, or on the tax total for the invoice.  This will open a window where you can adjust the amount of tax.

Greg,

There is no tax amount field on the Sales invoices, you must access that screen using Ctrl-Y or Sales, Tax Summary for line.

If you use the one at the bottom of the screen, it will print with the adjustment amount beside some text that looks something like "Tax not included 0.01" if you adjust the amount by a penny.

Once again thanks for that great tip Richard! - I have had to adjust just by the penny and disliked the way it showed on printed invoice.

That's interesting. I don't see any "adjustment" when I amend the tax amount at the bottom.  Notice the extra buck of GST in the example below.  The J/E looks normal.  Where do you see this "not included" note?  I'm on Sage 50 pro.

I'm entering invoices from a year ago.  I often amend that GST field to match what she actually invoiced the customers based on a desk calculator.  I've never seen an "adjustment" mention.

Hit Print or Preview the form if you can.

Thanks.  I've been entering invoices from last year.  Our invoices are done up in Excel, at least until I catch up with real-time.