I have a 9 month old Dane mix puppy with a hard mass in his toe?

Great Danes (and other large breeds) are very prone to osteosarcoma- bone cancer. It would be rare to see one this young, but it is possible. You are doing the right thing by having it taken off, your pup will do fine without that toe. You probably have about a 75% chance it's not cancer, but again it's very hard to say, especially being this young. Make sure that the biopsy states there are clean margins and that the doctor was able to get the whole tumor. It could just be a bony swelling, or some joint deformity.

Good luck

Good thing it's being taken care of so soon. The chances of it being cancer could be very slim, just depends on the dog. It could be a boney type mass, with great danes being so big their bones might not exactly grow in the way a smaller breed does. Atleast, no matter what it is, it's being caught and treated sooner rather then later. Good luck!