Vet screwed up MCT results

I have a 5/6 year old Australian Cattle Dog mix, and she had a lump on the front of her neck from years back (which was a cyst after testing when we first noticed it). A little over a month ago, a small lump appeared on the back of her neck behind her ear. We took her in and they tested both lumps and the front lump came back as a MCT, and they said the back lump was a cyst. They removed both lumps and sent the MCT lump in for additional testing. We just heard from our vet that they sent in the cyst and discarded the MCT lump. So we got no grading results, nothing, only that the doctor said she was confident she removed it all. They’ve admitted the mistake, and said the testing fee would be reimbursed and any surgery needed in the future would be covered. I go in to talk to the doctor who did the surgery in a couple of days. What do I say? What do I demand? I feel lost and heartbroken.