Why do you even have a btnReportPay_Click? That is not needed. You can add a command to your VM. In the VM, you know the payment (it's part of the VM). Then, you can do this:
Even better is to use services like this:
ReportService : IReportService which has a method ReportPayment. Then you can inject it into the constructor of the view model (all done automatically for you by catel).