GST not applied on negative item (rebate) in Sales Invoice

I am trying to include a negative item (a fixed rebate, not a % discount) in a Sales Invoice. Although the Unit Price appears correctly, the Amount column shows zero. Please can this be fixed or can anyone advise a way around this issue.

As soon as I posted the query above, I figured out a solution myself: create the rebate for a positive amount, but enter a negative quantity.

But it would be good if the original issue could be fixed. Thanks.

Negative amounts should work too. What date/number format do you have set under Preferences tab?

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