PO Contracts - Blanket purchase argreement

Dear SCM Gurus,

Earlier we were using BPA and the functionality was working fine for the client requirement. now there is change as mentioned below

Cost + 10 % margin . for example 1000 + 100 = 1100 is the unit price of a laptop.

While establishing the contract, 1000 is freeze, but while purchasing, if the price is different say 900,

The above calculation works for 900 900+90 = 990. how can we handle in agreements. ?

Another question, if I establish a contract for 1100 and release it with blanket release and receive it in oracle, get the invoice for 990. and pay 990, what will happen.

Costing method used is average costing

Regards,

Chaks