Oh wow OP! He’s being a right dick. Glad you can see that!
I’ve read all of your posts and the first thing that came into my mind immediately is that he can’t return or exchange it. His reaction is so overblown and irrational, it comes across like he’s hiding something. He’s kicking off like this so you’ll drop it and not push the matter, and if it upsets you and makes you worry about the marriage all the better as you’ll no longer be thinking about the necklace. He sounds horrible.
I reckon either he paid much much less than he’s claimed and doesn’t want to be found out (as in, it was a £30 costume jewellery piece from Accessorize not a £300 item), or he came by it another way entirely (he stole it, it’s an old item from someone else he’s regifting, anything really).
I bet you’ll never get it exchanged because either the seller doesn’t exist or the necklace never came from that stall or cost £300. It’s odd a jeweller doing well enough to be in a nice market selling items of that price would have zero online presence. Also odd they’re not there anymore. Also odd he won’t give you the card you haven’t seen proof of. It makes no sense to google the jeweller and nothing come up. Did your daughter even witness him purchasing it?
He’s being a real twat, wanting to reconvene in a week. I know you’re probably in shock but can you try and flip the power a bit in your own mind at least? Treat this coming week as time for you to really think about whether you want to be with someone who’d act like this. Don’t fall into the grovel apologetic mode or be waiting around to see if he ‘forgives you’, I bet you’ll never get the necklace exchanged and he’ll be frantically trying to think of a good excuse if at the end of the week you decide you still want it exchanged despite his strop.
It’ll be ‘she’s gone out of business, didn’t do as well this xmas as she hoped’.
I know I sound pessimistic but nothing adds up here.