
I've been on both sides of this situation and it's never a good one. I was interested in a potential show puppy, after of course the breeders picked, and at 8 weeks I went down, having been sent plenty of pics etc, and picked my one. 2 days later they emailed to say their pick now had a wrong mouth and they were keeping mine. Their prerogative of course as people generally breed a litter to keep one to show themselves, but they were pretty callous about it I must say.
Then in my last litter, I was dithering about which puppy to keep myself, and eventually offered the other one to a family, but then made the mistake of mentioning I'd decided to let that one go to another puppy buyer who had initially expressed an interest in that one, and after much thought I did decide to let her have her choice, as we'd built quite a good relationship, whereas the other family had only just come on the scene. They were understandably upset and refused to come and see the litter in case they wanted the other puppy, and all I could do was apologise. Completely my fault for more or less promising the same puppy to 2 people and lesson learned, though I do think if they were that keen on a health tested puppy they might have come to see the litter and consider whether they could form a bond with the other puppy!
Anyway, I do sympathise with your disappointment, and the breeder should have let you know there was a chance she would keep the one you had formed the attachment with. It might be worth going to see the litter and consider the other one you are being offered, but if you're certain that one isn't for you, all you can do is ask for your deposit back. I'm not sure where your post says they were being sold at 6 weeks, but if that's the case then get your deposit back and don't buy one at all, that's far too young.