How can I differentiate the merchants?

In terms of differentiating merchant, you can do so by state. In the first call, the authorization call, you set a state. MobilePay then returns this state, when you obtain the authorization_code. This way, you can differentiate, if you have more merchants working on the Consent process. Afterwards, it is your responsibility to manage, which tokens belong to which merchants. This is solely a security aspect, that we do not enclose information about this, when you get the refresh_token. But this is also a response for a call, that you initialised yourself, and the response comes instantly. This is why there shouldn't be any confusion.