Hi. I've been 'lurking' here on the Knot for awhile now do to helping a few friends of mine with planning weddings and hopeful soon my own

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First I have to say the Knot is a wonderful place to get wedding info and ideas.
Now for an issue that I have run into.
The 1 wedding that I'm helping with, I am also in the wedding party. The wedding is in Nov. I went and looked at BM dresses a few months ago (end of last year) and found one that I really really liked and the BF who went with me said that it was the one. (The bride said that the BM dresses have to be red and floor length.) While I had the dress on, the BF took a pic of me in it and I sent it to the bride asking to make sure that it would be fine. (I like to double check stuff.) The bride loved the dress and said it was great to go ahaead and get it. So I got measured and I was told that in that particular dress I would need a size 16. (At the time i wore a street size 12-14.)I questioned them on the size 16 and they showed me the measurements and said that the specific dress along with a few others the sizes ran smaller for some reason. Ok. Since I was out of work and once again out of work and knew it was going to take a good while to pay for the dress I had them "order" it. They rang it up and that and I put down $20 on the dress. The told me that the dress wouldn't actually get ordered until 55% of the dress total was paid for. Ok. No problem I have time. I got a phone call not quite 2 months later saying the dress was in!!!!!!!!!!!!!!! I went in to make sure it was the right dress try in on and question them as to why the dress was ordered. I try the dress on it's a size 18 not a 16!!!!!! so I grab the attendant and told her the problem who then went to get the manager and told me to not change out of the dress yet so the manager could see it on me. The manager comes and looks at the dress and sees that it's a good 3-4 sizes to big on me. Tells me to change out of it while she looks the records up to see what happened. Admitted that it was their fault on wrong size and ordered at wrong time. They knocked about 10% off of my bill. Alterations at this point were stiill doable.
Since that time I have lost about 10 pounds. OK, no big deal really except I have gone from a (street size)12-14 to a size 6 possibly even a size 4!!!!!!!!!!!!!!!!!!! I don't know how or why but I've just dropped a lot of sizes. I'm afraid to go back and try the dress on now cause I know how bid it was last time. I was starting to swim in it. What do I do? The BF's mom is going to do my alterations on whatever dress i get. I don't know what to do. I wasn't planning on losing any weight let alone that many sizes. The sales receipt says all sales are final. Do I continue and finish paying for a dress that's way to big and possibly past being altered and try and sell it if it can't be altered and get a different dress? Do I see if they can somehow let me exchange/swap or whatever for a different dress? Help!!!!!! Has anyone had any kind of issue like this before or have any ideas? I'm open to all help and ideas right now. Sorry that this was soooooo long. I really wanted to try and keep it short. Thanks!!!! here's a pic of the dress. BTW the beading is very elaborate and the dress has been discontinued.