Betgra, you HAVE to ask permission for unique offers, period.
Then you will be made a deal for them.
888 is not obligated to honor any agreement with you because there is none.
They have decided to pay you what they see fit for what you sent. Without any agreement. They made their analysis of the traffic you sent and based a reimbursement scheme on it.
Take it. Others might pay nothing at all without agreement.