How do I manually change the negative tax amount in an expense?

How do I manually change the negative tax amount in an expense?

Hi,

I have a  Tax Group with a  (positive{IVA} and negative {IRPF}). Now have to ajust some IRPF tax manually but I cannot write manual negative value. How can I do manually change the negative tax? 

https://www.zoho.com/books/kb/taxes/tax-override.html

Thanks,

Oriol