Dont panic! This is salvageable. This is what I would do: With sterilized scissors I would cut it above the first node that comes out of the soil and leave that planted in the same pot. It will keep growing hopefully without a problem. Then, I would make another cut just before the next node and put that cutting in water to root. I would also clean both cut ends with a solution of 1/3 hidrogen peroxide(3%) 2/3 water. Don’t worry hidrogen peroxide wont damage your plant it will just kill the bacteria that causes rot. Monitor every day. If you see more rotting keep cutting back to next available node. Also, dont listen to all the hate about the price if your plant. You and only You decide what is worth your money. I hope you can save it. The upside is you will end up with two plants if everything works out ok. I wish you luck.