Prestashop- How to add Amount Due in AdminOrdersController.php

My Prestashop version is 1.7.5
I wanted to add the amount due value in the AdminOrdersController. Amount due value can be calculated using (‘total_paid_tax_incl’-‘total_paid_real’)

        'total_paid_real' => array(
            'title' => $this->trans('Real Paid', array(), 'Admin.Global'),
            'align' => 'text-right',
            'type' => 'price',
            'currency' => true,
            'badge_success' => true,
        ),

So I modified it like this

        'total_paid_tax_incl'-'total_paid_real' => array(
            'title' => $this->trans('Amount Due', array(), 'Admin.Global'),
            'align' => 'text-right',
            'type' => 'price',
            'currency' => true,
            'badge_success' => true,
        ),

But It is throwing an error, Warning: A non-numeric value encountered.
Though both value ‘total_paid_tax_incl’ & ‘total_paid_real’ are in decimal(20,6)

Can you guys help me to solve the issue?