So, I just came back from the store.
Poor thing didn't make it over the night.
I talked to the fish manager who took care of the fish, he was extremely knowledgeable and definitely knew his stuff. He said he didn't just have Velvet but apparently suffered from internal parasites as well when they did a more thorough check up on him. He understood my concern and told me the same thing about not telling them if I plan on taking a sick fish that the fish itself is sick.
They treated him for fungal infection but in the morning when they checked on him, he was dead. We actually had a long conversation about fish and fish diseases too and he mentioned his own disappointment. He also informed me he doesn't mind when people are persistent about information and stuff about fish because he's actually glad there are people who take more invested time into things.
So, it's a learning experience, but in the end I wasn't able to save him. Even if I did take him in that day, his chances of survival would have been terribly low still.