Remove the lump and then test it??? Have they done a fine needle aspirate on it already to confirm that the lump is cancerous? If not then that's the first step IMO.
Trixie had a lump at the start of the year. We left it for about three months to see if it grew or went away. It didn't do either so we had it tested. The results were inconclusive, but they said that it appeared to be a mass cell tumor and recommended that we had it removed.
She went for surgery shortly afterwards and we elected to have the lump tested. It turned out to be a Grade II tumour but the margins were clean, which means they got all of it and she had a clean bill of health. She was utterly miserable after the surgery. We had trouble getting her from the clinic to the car and then into the house. She was in a lot of pain. We put her straight in her crate and she stayed there for about 24hrs. By choice. When she finally got out we let her on the sofa to keep her away from Chopper and Angel's licky tongues.
She took about a week to recover. They took her stitches out after fifteen days. I don't have a picture of her back today, but these were taken a few weeks after. The hair has grown back, but her scar is still quite visible.
I'm not sure exactly how long it's been since her surgery, maybe 8 months? But, she's totally fine and hasn't had any more health problems since.
Fingers crossed your dog's proceedure will go as well and will recover as quickly. Please let us know what the prognosis is? Try not to worry. Dogs get lumps and bumps removed all the time