Wordpress Product type

Im trying to sort out how to make custom product type with following options:

Im Creating item #B, on item #B i want it to adjust the warehouse amount on article #A because they are the same article but its a bulk pack.

Item #A = 1pcs Item #B = 20pcs of Item #A to a better price.

Can i multiply below with a custom field with the amount and then remove it from my other custom wield with the articlenr?

$the_product->stock_status

How can i manage to build this or does anybody know about this type of plugin, i have searched the web for a couple of days.

via Chebli Mohamed