Exact same thing happened to me!!! I was fitted with an XL Seaquest Pro QD+ by the owner of my LDS. Although I originally wanted the Balance, he only had a medium in stock and said they were on back order indefinitely and proceeded to convince me the Pro QD was better in that it had pockets and was jacket style. Being wide-eyed and bushy-tailed, I took him on his word as I did with trusting him that the XL was the size for me although only a couple of inches of the Velcro cummerbund made contact. He claimed that would allow for a dry suit one day which I had already expressed that I wasn't interested in dry suit diving.
After dive one, I new there was a problem with this BC. It would ride-up on me and at depth with compression, I couldn't tighten the cummerbund any tighter with a 5mm wetsuit. I returned to the LDS and had a 4-hour debate with the owner regarding the fit. Like all good crooked salesman, he convinced that it was my body shape (pretty normal... 6'2" 210lbs) and that all BC's would ride-up on me and my only solution was to get a crotch strap... Funny thing was, the I'll fitted BC he sold me didn't allow for a crotch strap!
I then sucked it up making whatever adjustments I could and proceeded to dive it about 60 more times. Then I took about a 6-month sabbatical from diving due the birth of my daughter and moving across country. After the move I dived it at a local quarry and had enough of it riding up on me! Determined that it would be on eBay the following day, I mentioned my frustration with it here on SB. Since it had been over a year since I had purchased it and I now lived a couple thousand miles away from the original LDS, my options were limited. A fellow SB's who used to own a shop and sell Aqualung suggested that I call Aqualung directly and explain the situation. Aqualung agreed that the owner failed to honor their 30-day trial period and allowed me to exchange it directly with them. Ironically, I ended up with a medium Balance! What I could have had in the first place without all the hassle and miserable dives fussing with an ill fitted BC. Big props to Aqualung for making it right. Shame on the LDS. This is when I question the benefits of paying 30%-50% more with buying from an LDS.
Sorry for the rant but wanted to let you know your incident wasn't isolated. Like someone else said, I think he needed to get rid of the XL for what ever reason in both yours and my case. Nowadays, I trust SB and all the contributors for their unbiased opinions and never ask advice of an LDS employee who I feel the majority of are full of caca with less experience than I have. Sorry if you're an employee and this offends you.... I haven't met every LDS employee so my previous statement is of the few I've met. I'm sure there are some great ones out there.... finding you is the difficult part. YMMV