Allowing negative priced item within invoices

For clients in various part of world, it becomes necessary to capture information of Advances/Returns/Buybacks within their invoices. This help them for legal purposes and to provider finer details inside invoice

What's New

Inside invoice we allow customers to create product on the go. Here we allow the customers to have negative prices
Products with negative prices won't be available to save for later use nor they can have taxes assigned to them as this might break existing apis in places using products
The calculation done here will be same like it used to be so far, with just negative prices subtracted from final total

Why It Matters

Increased flexibility - Allows user to be move creative while creative invoices.
Stay compliant - Simple, intuitive yet effective way to stay compliant!