Hans Christian Andersen's The Little Mermaid IS the original tale. There was no version of it before his. Disney departed from it HUGELY.

You might be thinking about the novella, Undine, written in 1811, which also features a mermaid/water spirit plotline where the young female mermaid/water spirit gains a human soul. In that story, she gains a soul by marrying a human. Hans Christian Andersen was influenced by that story in writing The Little Mermaid, but his story is original.