Parents won't let me sell my ferret?

So I asked this question before but I didn't leave enough information so I'm asking again. So I wanted a ferret and after about three months my parents let me get one but I had to pay with my own money and I also had to buy the cage and the accessories (which is a lot of money for a 13 year old!!) but my parents agreed to buy him food. I thought that I would be allowed to play with him through the house but he isn't allowed inside at all, no matter how much I ask my parents they only let him in my room which is very small so there isn't much to do with him. I got one because I was told that I could get another one if the first one went well but my parents changed their mind and I am only allowed one. I want to get another one for him as he is in his cage all day when I'm at school. The only thing I do with him is walking him around the house as he isn't allowed inside and he does enjoy that a lot but it does get a bit tiring and repetitive only being able to walk him around my backyard and whenever it's raining he doesn't get to go out of his cage since he can only walk. It's not fair on him to be alone all day and we can't exactly bond as he only walks around but every time I say I want to sell him I get told that I wanted him so bad so I can't get rid of him and that I'm selfish. Please tell me what can I do so I can sell him?