Delva still run his other business enterprises, so his "gul salary" isn't the only income he has. I think he also enjoys prestige of officially working for the Cardassians and this is a good argument when starting a new deal with some Cardassians.
In a Cardassian library
or in a Cardassian gallery
"Reagan, it appears, is really only an ardent unionist if the unions in question are in Poland" - Stephen King, Skeleton Crew