I think you need to tell the supplier to send you a replacement cutting. Send this picture you posted and tell them it is not going to make it.
The rot is progressing upward and it is hard to root a shorter cutting. And even if you can successfully root it. It takes twice longer for it to bloom since it has to work harder to get to the mature size. (It's almost like starting from seeds.)
Most reputable suppliers will send you a one-time free replacement. They want your repeat business.
