They Gave It To Me So How Is It Theft?

I posted this in a different forum right when t happened and this is the version with updates. I buy storage units. On Saturday I almost finish emptying a unit but planned to come back the next day for a huge couch and a couple big items. I called the next morning and the manager said I’m in trouble and i owe them a couple hundred dollars because I ‘took’ the wrong unit. He walked me to the unit unlocked it, said it was mine and helped me at different times through the evening. Turns out he gave me a more expensive unit (that was also on auction and won by someone else who came to claim it and said that the one I bid on was not what he bid on) which looked a lot alike and I had no idea until he told me.! Then I get a call from somebody who says they’re district manager or something like that and tells me that if I keep the stuff is considered theft and do I really want to go that route. He said there is no way I didn’t realize it. I explained that the size of the unit doesn’t make it better, I’ve found silver in a $20 unit and dirty clothes in a unit worth $1k. He asked if everything was still intact which of course it was not because I spent all night unpacking and he wanted me to pack everything back up and bring it down to them. The company that run Auctions and they said that’s not my fault at all, and I could keep it if I wanted to according to them, they wouldn’t hold it against me at all, I’m just wondering is it considered theft if somebody gave you something told you was yours and you keep it? He was the manager he does all the auctions. He has been there for over 2 years. Someone else told me there are so many procedures in place it makes no sense he gave me the wrong units and that in the 17 years they have been doing auctions they never seen this. I offered to pay a little more towards the unit but not the complete amount since they are taking my time away from working and holding me up from purchasing other auctions since they have my money. They said no, only for $50 more than I offered. I personally think it is due to what is left inside the unit. They technically cannot re-auction what I already went through, it’s tampered with. And any business man knows the time value of money. It’s not worth it for $50. I left him a message today saying he has 48 hours to pack things up and clear it out but I expect them to come with an itemized list so they don’t grab my stuff. Am I doing anything illegal? They made the mistake. Mine was forgetting my glasses at home. I don’t want to give it back at all.