You could try telling them you want to try and rescue him, and that you know there is very little chance of him surviving, so you're not paying for him. I did this at Walmart, where I rescued Finn (RIP). The lady took him out of my hands and said this fish is dead. When I pointed out to her that he was in fact not dead, but very very sick, she said just take him and good luck.
You could try convincing them if you're up to the task. But as far as space goes that's another story! When I grabbed Finn they also let us walk out with a $4.99 1/2 gal tank...for free.
Feel bad for that poor baby!