I have figured out how to do this. In the payments section, if the total amount owing is $300, and the customer pays $50, you'll need to not only input the $50 that was paid but the $250 as unpaid. That clears that up.

that's correct. the "pending amoun" is to total summatory of pending payments (not the total of the invoice minus the summatory of paid amount)

This is not practical at all. not how any of my other invoice apps worked, and now I paid for a program that is actually making us a lot of manual effort since we have to now put in payments beforehand else it shows the user owes us 0. But we do not know beforehand what a payment will be. and if they only pay partially we would have to amend that payment and ad another.
Why not have it work like other apps where outstanding is invoice minus payments submitted as paid. Now we have to go manually and create payments outstanding beforehand for the outstanding to show correctly. This is so not practical. How do I fix it, or what will it cost me for you to change it? Kind regards :-)

My solution, a one-liner :-) See image attached.

The fix! Go edit the file --> /components/com_invoices/views/invoice/tmpl/default.php at about line 363: $amount_outstanding = $total - $subtotal_payments;
Then whenever there is a pending payment manually entered or not the outstanding amount will at least correctly show the invoice amount minus the payments made.
I hope this helps :-)