CallbackAddress information missing - help wanted

[IgorBalog-Eng]

UseCase provider posts offer right arrow:

• provider posts ContractOfferMessage to the consumer (https://consumer.com/negotiations/offers); message contains provider callback address https://provider.com/return_path/ for example)
• consumer accepts request, process it and returns ContractNegotiation.OFFERED
• consumer review offer, and for simplicity, accepts it
• consumer sends ContractNegotiationEventMessage.ACCEPTED to the provider (https://provider.com/return_path//negotiations/consumerPid_id/events), using provider callback address from offer request
• provider recieve event message, process it, and return 200 OK; contract negotiation status on both sides is ACCEPTED
• provider creates ContractAgreementMessage and sends it to the consumer https://consumer.com/:callback/negotiations/:consumerPid/agreement

Problem! how can provider know which is consumer callback address since this information is not send by the consumer in any of the previous steps? Callback address can have that :callback part, which is unknow for the provider to be able to proceed with contract negotiation. Event message does not have callbackAddress so it can be used in following calls. Provider cannot know what is :callback part so it can somehow construct agreement URL from first (ContractOfferMessage) request.