the deal is the same .the mermaid can only remain a human if she find true love's kiss the prince falls in love with marries hers. however.the penalty in the movie is only that ariel will turn back into a mermaid if she falls.In the story she will die if she falls.also.while the prince remain a main motivator, the mermaid in the story also motivated because human have eternal soul,and mermaid do not