I assume dell doesn't list a mac driver on their website.
I'm pretty sure the Dell A920 is a rebadged lexmark x1150, one could
try the
Lexmark drivers.
*note* I have no direct experence with the dell a920 and any mac. My
experence is limited to telling people how much the ink costs and
spending $100 on another printer if they are printing more than 1000p a
year would be cheaper.