Apple Store screwed up Apple ID on iPhone 6 Plus and now it is stuck in Activation Lock and they won't help me fix it.

I took an old iPhone 6 Plus to the Apple store to have it wiped and put in factory default mode so that I could give it to my granddaughter for her birthday. They hooked it up to a laptop and did that. It looked like it was ready to setup so I turned it off so she could set it up when she got it. When we turned it on it has a Activation Lock with a strange Id associated with it and no longer knows my id. I took it back in and now they won't fix it for me unless I bring them a proof of purchase which of course I can't find because I bought it in 2015. It was associated with my iCloud account until they screwed it up. Anyone know of any other way to fix this?

Thanks