The original Hans Christian Andersen version of "The Little Mermaid" has been called the female counterpoint of the Faust tale, though the Disney adaptation retains many of those elements as well. In both stories, the protagonist sells something of immense spiritual value to a sinister figure in exchange for what they perceive as a better existence. Faust wants power and youth. Ariel wants to become a human so she can both explore the dry land and be with Prince Eric. Both characters suffer as a result of this deal: in Marlowe's play, Faust is damned to hell, while in Disney's The Little Mermaid, Ariel comes close to both losing her prince and losing her freedom.
Both characters could be said to overreach, but it is clear that Ariel is meant to be a much more sympathetic character than Faust. Faust is selfish and power-hungry. His desire for magic and youth come from a desire for power for its own sake. When he gains his desires, he only wastes time, pulling pranks and romancing illusions such as the phantom image of Helen of Troy. Any noble intentions he had for the power are ignored the moment he receives them.
In contrast, Ariel's desire to see the human world is framed as reasonable: her extreme decision to cut a deal with Ursula is prompted by her father destroying her collection of human objects, a symbolic rejection of her individuality. While it is unwise to deal with Ursula, Ariel is naive and hurt by her father, not making a consciously immoral decision as Faust is. Even though he knows he will be damned and that he is in the wrong, Faust sticks to his bad decision until it is too late to change. Ariel repents when she sees how her actions have hurt her father and endangered Eric. It is telling that Ariel also gets to have a happy ending, while Faust does not.
In each story, Ariel and Faust give up something precious: Ariel gives up her voice while Faust gives up his soul. While Ariel's voice does not seem as precious an asset as a soul, in the context of the Disney film, her voice represents her individuality and agency. She is unable to express herself, create music, or fully connect with Eric without it. In essence, she has yielded her power, reducing herself to a "pretty face" as Ursula puts it in "Poor Unfortunate Souls." It is essentially the secular equivalent of an immortal soul, since Ariel is separated from beauty and love when she no longer has it, just as Faust is separated from transcendence and the love of God when he relinquishes his soul.