I’m not sure about how payments work now. Everything is right when you introduce manually the quantities, but, why is it possible to force status to paid if payment hasn’t been introduced? That creates some inconsistencies with the data, as is not really paid (in the system), but it’s marked as paid.
Edit: I think “paid” status should be marked automatically when payment is complete without having the chance to force manually.
I already noticed this but we can’t automatically set the status and disable the manual option as there are users who don’t use the payments, only the invoices.
We could make this an option for the settings but nothing more. 
Yeah, that’s what I meant. If you have the option you can use one system or the other, but not both combined. If you don’t use payment system, no need even to see those menus, but if you’re using it, the program shouldn’t be able to let you mark as paid.
Anyway, maybe is better just consider it for the next version.