Validate an invoice means the amount of that invoice will be passed to accounting/finance/invoice modules.

The designed workflow is having, for example, Sales, can have several draft invoices, so saving and editing are needed.  When the final version is settled, you can have either a manager or whoever else to validate the invoice.

My suggestion to your needs is to do it over python with a custom module.

Create a custom module adds a new button or field at desire location.  
Create a new action links to the button or @api.onchange function to check/change fields.  
Based on the previous step, generate data or config file for your API to test it.  

If you need Odoo to show the API testing result back in Web interface, I think it depends on feedback your API can provide.