Finish Cassie's quest first.
Push aside the Slum pass guard, or trick him, you do you.
Sleep return to the slums and you will be brought to the Don, do "Grunt Work" not horny work.
And visit Cassie, afterwards.
https://lustdollplus.miraheze.org/wiki/Cassie%27s_rescue
After all that ask Arlene, the fish lady for a session.