Sounds like you will have a tough time giving him back. I'm like that, I now have 4 hamsters (2 I got by accident as I offered to look after them till they are re-homed
)
Have you got Aldi near you? I have seen a big (25cm) hamster ball in there for something like 3.99. I just thought I mention it just in case something unfortunate would happen to his original ball - like fall down the stairs and straight out the front door only to be run over!
Or, what you can do is go to the pet-shop and buy the biggest wheel for him you can find (even the noisier once can be quietened by putting some vaseline or a sellotape on the easel)
Good luck with the looking after and maybe post a pic of peanut for us to see!