Prevent invoices which are paid by invoice from being marked as paid when order is completed

functions.php

<?php

add_filter( 'storeabill_woo_order_is_paid', function( $is_paid, $order ) {
    if ( 'invoice' === $order->get_payment_method() ) {
        $is_paid = false;
    }
    
    return $is_paid;
}, 10, 2 );