to promote my app I would like to make it temporary (48 hours) from a paid to a free one.
Can I make it a paid app again after it was free?
No, this is not possible.
You need to make your app either free or paid, it is not possible to switch between the two...
Just to get it right - Once it was free app, it must stay a free app forever and can't be turned into a paid one again?