What is a "fiche"? Do I need one for Mauritania?

Many forum entries speak of a "record" for Mauritania and that they needed dozens to deliver them to police checkpoints. What are they and can I travel to Mauritania without them?

In what currencies can I pay the visa upon arrival from Mauritania when entering by land?

I am currently in Agadirm Morocco and I plan to enter Mauritania by bus. Opinions about Visa availability on arrival at the border differ, but I have not found someone who has returned there so far. Online I can find visa prices on arrival of 55 euros / 65 dollars.

Does anyone know if they also accept Moroccan dirham?