I all was just called up or wrote to the dealer and told them "Hey you made a mistake" Most dealers will make up for it so it doesn't hurt them. Even dealing with T's a person will tell 100 people something bad about a dealer than they will say something good. Dealers know this. But that is just my thinking.
Particularly since it's also only been a month, most dealers would try to make good for their mistake. I would make a polite phone call explaining the situation and see what they say. You might get offered an exchange, or even something as good as "keep the spider, we'll issue you store credit for your next order" if it's not in their interest to take in a mature male.
Of course, others stand by the notion that sexing is an imperfect science with their busy schedules and nothing is guaranteed. If this is the case, come on here again and spread the bad news Can't be good for the dealer.
I'm not sure if anyone remembers, but I bought two spiders at a reptile show a little bit back, and was told they were female. Both were male once I got a look on my own. I emailed the dealer twice, but they refuse to email me back. Great customer service, eh?
If you buy spiders from someone who mainly deals in reptiles, chances are they don't know enough about them to sex them. Many reptile dealers don't relize that the tibular spurs are only visible on MATURE males, for example, they may tell you it's a female because that's what they think. Others may simply tell you what they think you want to here to make the sale. The honest ones will tell you they don't know, but most won't guarentee the sex. If you want a guarentee, you need to go to a specialist dealer, and then you can expect to pay more, and rightfully so.
I like to buy at reptile shows, as it gives me the ability to examine the animals before purchase. But, buyer beware! Chances are, you may not be able to track that person down again.