How to restrict BPA Release based on quantity agreed

I need to achieve following functionality using form personalization:

 Quantity Released + shipment quantity in current release must not greater than Quantity Agreed mentioned in the purchase order.

 For example,

in my po following are the details:

Quantity Agreed - 100

Quantity Released -80

Now when I enter the quantity in release, system will give an error message.

So I cannot enter more than 20 here.

 How can I achieve this using form personalization

Thanks,

Senthil