testing here... https://test.idempiere.org/webui/
(I dont know how to find out if this is already reported so aplogies in adavance if this is a duplicate)
add an invoice (vendor) for $1000 for the Chrome BP
create a Payment Selection that selects the invoice.
change the Payment amount to $500
create the payment.
create a new Payment Selection
use the Create lines process and select invoices for the Chrome BP.
the unpaid balance should be picked up in the Payment Selection Lines but is not.
To state the problem more generally, it appears that after an invoice has appeared in a Payment Selection Line then it can not be made to appear in a subsequent Payment Selection regardless of whether the invoice is unpaid or part paid. It makes sense to avoid selecting it twice and possibly causing a duplicate payment but the current code seems too restrictive.