Update (08.04.2016 04:20 pm):
The problem is solved.
You will find your invoices with the right currency in your personal area in the section "My Payments". Just open the menu of the course (marked with three dots) an choose "Payment Overview".
Right now we're experiencing a technical problem that posts the wrong currency on your invoice. But the actual billing will be processed in the correct currency, shown during the purchase.
We're working on a solution and make an announcement as soon as the correct invoices are available via your personal area.