Disappointed Loyal Customer

I bought a 7kg Cadac Gas bottle from builders express and when I got home, I realised it was the wrong size so I phoned them to ask if I could return it and they said it would be no problem, they just won't be able to refund the gas. 2 days later, I took it into the store only to be told yes, I could return the bottle in exchange for nothing, no store credit or money back. This honestly ridiculous as I had enquired the previous day. Very dissapointed by this as I have bought from this builders many times. I will not be supporting them anymore.