First of all, I wish to thank you all for your helpfullness. I read arabic but I'm just used to printed text, so it's really difficult to me to decipher these inscriptions. By comparison with the fals kept at Zeno Database (I'm pretty sure that's copper) I tried to decipher it. I hope it's not just a bunch of wishful suppositions, and I'll be very glad if someone more expert than me and/or an Arabic mother tongue speaker could help.
The reverse should say
"La Illah Illa
which means "There is no god but Allah, alone, without partner", pretty commonly found in different Umayyad and Abbasid fals.
The obverse should say:
"Mimma amara bihi(?)
Which means "By what was ordered by al-Mahdi Muhammad, commander of the faithful, by God". I'm not really sure about this interpretation. For a comparison, this face looks similar to the obverse of this coin https://www.zeno.ru/showphoto.php?photo=50109
I can't see a date or mint (maybe it was on the circular margin, which seems to be worn) but, if my interpretation is right, it should have been struck during the caliphate of al-Mahdi (775-785). Thanks a lot! Waiting for some feedback in this fun quest!