If you bought the item from a local store, got home, saw it was not correct, would you call them and tell them to refund your money right this moment with your promise to show up and return the item?
I've run my own shop and unless I have the returned item "in hand" the buyer gets nothing. I've also had stuff show up on my shelves that has clearly been used when delivered from the distributor/warehouse in a fresh package. Crap happens.
I don't understand leaving neg feedback for something that you are in no worse position, other than time, than when you started. The seller has offered to square up, pay return postage, where's the harm.