The pictures don't look like a tumor at all, which is good!
Either a) It could be him poorly bred to where he has a deformed lip b) He has dead tissue on his lip from lack of proper care c) It is very badly burned from the ammonia, like the rest of his body.
I honestly think if you keep up these very frequent water changes and keep everything as natural as possible, he will heal up fast! I am glad you went for IAL compared to salt, salt could damage his system very quickly in the condition he's in. Keep taking OFL's advice, and diligently work on healing him, and I think he'll make it!
I just love his tail colors, they are stunning. I can't wait to see him in a month or two when he heals up and his coloring comes back again!
You should definitely keep taking "before" pictures, then take pictures when he is healed and colored up, then go back to the pet store and show him the pictures. Tell him he needs to keep them in proper water/housing with good care so they keep their vibrant colors and they appeal to people more so he can actually sell them for a good price. Hopefully he'll listen to that!