I have billable expenses which on purchase have 0% VAT and are entered as such. On invoicing them, I need to add 20% VAT but that changes the expense VAT. How to fix?

Hi there, @katesiraj.

You can change the setup of the invoice to Inclusive of Tax when liking a billable expense. I've replicated this scenario and checked the expense transaction if the amount was changed, and found out that it's still the same. 

If the setup is correct, I'd suggest using an incognito browser when entering this transaction to check if this is a data-related issue. If the details are correct, you can go back to the regular browser and clear the cache. This is performed to delete those outdated data that may have caused this issue. An alternative way is to use another supported browser.

Here are the keyboard shortcuts to access a private browser:

• Google Chrome: Ctrl + Shift + N
• Safari: Hold down ⌘ + Option + P
• Microsoft Edge & Mozilla Firefox: Ctrl + Shift + P
  
  

I've added this article for your future reference about the VAT codes: Common VAT Codes.

Please don't hesitate to let me know in the comment section below if you have other questions or concerns. I'd be glad to assist you further. Take care.