People knowledgeable in Qur'anic Arabic provide evidence that 12:20 is a prolepsis?

Qur'an 12:20 says regarding Joseph, "And they sold him for a measley sum, a few dirhams counted, for they little valued him" but dirhams didn't exist in his time. However, a Muslim I'm debating with argues that it could be better translated as “And they sold him for a measly sum, a few dirhams counted were how little they valued him" and thus it was a prolepsis. Is there evidence of that in the original text?