Ask the original artist first.
I don't think they'll have too much of a problem as long as you detail out that you will not be generating any profit from them (unless you're on the Ad Revenue program. Then you'll want to mention that).
If you're in the US, this can come in handy: https://www.rocketlawyer.com/document/license-agreement.rl
Another option is to buy all the rights to the character, which I believe this is out of the norm for many people who make adoptables.
Unlike a commission, you are buying the right to use this character for anything you wish. Which means ONLY YOU are allowed to seek legal action against anyone else using that character. ONLY YOU are allowed to legally create merchandise in this character's likeness. ONLY YOU can legally grant permission to anyone else who wants to use your character in some shape or form.
(If the character design is used as a palette-swap adoptable, the artist will not be able to use recolors of that design for profit anymore, as recolors infringe on copyrights.)
I believe $300 would be the ABSOLUTE MINIMUM you can offer when buying rights to a design/character (like a simple chibi. Regular proportions would be $500+). And even then I feel like you need to pay some more :/