The user cannot cancel their connection with the merchant through MobilePay. They can solely disconnect their payment method. MobilePay is not a part of the agreement that the merchant has signed with the customer. The merchant will receive callbacks real-time, in case the customer unsubscribes to automatic payment in the MobilePay app. By default, the cancellation takes effect immediately. Afterwards you should clarify with the customer, how upcoming payments should be handled. You can still offer the customer to pay with MobilePay; the customer just needs to connect to MobilePay Subscriptions again.
The customer can see the payment as a future payment at least 1 day before DueDate. If the customers wants to be notified and the merchant has agreed to this, the customer can be automatically notified the day before, the payment is executed.