Reversing sign on figures is the easy part but your current issue cannot be solved by reversing sign. Working on that.
Also, as for tax code naming. When using your tax codes on invoices, you get these subtotals. I don’t think that’s right. Shouldn’t it say VAT 9% instead of 9% Sales ?
You can configure 9% Sales tax code like this:
This will show VAT 9% on printed invoices but 9% Sales on forms.