Facebook is set to allow friends to send each other money using its Messenger smartphone app, it has been revealed. The feature - which is yet to be activated - was discovered by Stanford computer science student Andrew Aude with the help of app exploration developer tool Cycript. The option lets users "attach" cash to a message in the same way they would typically attach a photo. The app uses any card details registered with Facebook by default, or they can add a separate card. Facebook could charge a small fee for money transfers if it wanted to make money from the app. Alternatively, offering it as a free feature could be a way of driving new users to the software. Earlier this year Facebook poached former PayPal president David Marcus. Clues hinting at payment capabilities in the app were first discovered by security researcher Jonathan Zdziarski last month. The demonstration by Andrew Aude shows that users will be able to hit a button to initiate a payment, enter an amount, then hit send. The transaction is private and details of it are not published to the news feed. In April Facebook pulled the chat feature from its main app, and told users to download the Facebook Messenger app to continue using the feature.