Only allow items on sales order if they are on trade agreement

I run AX 2009 (kernel 5)

So, I have created an Customer Price group. (Accounts receivable - Setup - Price/Discount)

I have inserted a bunch of items to the trade agreement for this Price Group.

I have on the customer details choose this Customer price groups for some of my customers.

When I create an sales order with the customer and with the items on the price group every thing works beautiful, the prices are the ones I have inserted in the trade agreement. 

Now what I would like to do is to block (or give warnings) if a user in a sales order attaches an item that is not included in the price group which is attached to the Customer.

It this possible? I don't mind workarounds or small warnings, anything will do.

Regards