Just to clarify for OP or anyone who can answer, I don't think Ashur was portrayed as a bookkeeper or accountant in any way if I remember correctly. He was kept after his gladiator career for performing general errands, carrying messages, accompanying Batiatus to aid in transactions in the city, as a reasonably capable bodyguard if needed, and most importantly as a thug for Batiatus' less than legal needs and as eyes and ears around the ludus. As far as I can remember he was shrewd and unforgiving, not educated.