The Finger (Futanari Experiments #1‪)‬

When Brie receives a large, attachable "finger" that enhances sensation, the first thing she does is stick it on her c**t. When it works as planned she proceeds to use it and abuse it, then shares the fun with her best friend and co-worker, Erica, who went to investigate why Brie missed an entire day of work.

This story contains explicit depictions of sex between two women with the help of an attachable, feeling "finger."