How do you remove black dye from a sofa cover off a grey sofa?

This happened because you did not set the dye on the cover first. These might help.

https://www.leaf.tv/articles/how-to-remove-dye-from-a-leather-sofa/

https://www.hunker.com/13423448/how-to-clean-dye-transfer-off-leather

https://m.wikihow.com/Remove-Dye-from-Leather

Some of these seem downright hinky and damaging to the leather finish, nail polish remover? I think not.

I would go to a tack shop and get saddle soap and a good leather cleaner and conditioner. Spending a few dollars to save a sofa that probably costs thousands seems reasonable to me.

I know this might sound weird , but did you try magic eraser ? I have a light beige leather sofa , and I use magic eraser to clean my sofa . Please try this in an unseen area to test it , to make sure it doesn't pull the color of your sofa off as well .

Another strange idea ... I also use an amazing carpet cleaner call folex , I found it at home depot for about $6 . This gets everything out of carpets , ink , wine , rust ...It works so well that I keep some in a spray bottle in my laundry room . So maybe put some on a day and gently try wiping your sofa , also try it first in an unseen area to make sure it won't pull the color out of your sofa . Good luck !!!!

Kellie,. Try using Saddle Soap, it just for leather. Find it at an automotive store, call first. Follow djirectioons for stain and cleaning. Aloha!