Need to Register sales between 2 Websites

I have a Deals Aggregator website, we are establishing partnerships with companies, and we need to control the sale everytime a user purchases a coupon. I need suggestions and solutions of how to control this throw webservices or other option so both parties are able to know how many sales were done to present a report in the end of the month. Please check attachment for explanation. Let me know if you have any questions.