We cannot depend on LFS giving us correct info about puffers, 99% of the time. That's what this forum is for. If purchased before researching, the next right thing to do is make sure the fish has the proper housing & if not, return the fish. It will be much more difficult to return when it is larger, especially if you've grown attached to it.The LFS is to blame where he purchased the MBU, not him. He went in and more than likely fell in love with the Puffer like most of have done, purchased it, then came here when his LFS had no info to let him know.
Your telling me every single purchase you make in life you research to the fullest extent?
Please be sure to read the article stickied here on the mbu & what requirements you will have to afford for it in the near future. They grow around an inch/month.