A secret deal between Google and Mastercard is allowing selected US retailers to see when online ads result in sales at physical stores, reports Bloomberg. Following "years of negotiations" Google paid millions for access to Mastercard data and then gave retailers access to a tool called 'Store Sales Measurement' which enables them to track the link between their online advertising and purchases made in store. Both companies are insistent that no personal data is being shared in what they call a beta test of the tool. Advertisers see aggregate sales figures and an estimate of how many are related to Google ads placed by the company.