Well seeing as how this is a mistake on the LFS's part, they would hopefully be flexible in terms of maybe just exchanging him for a REAL bristlenose or another (more suitable for your tank size) fish? I think your best bet would be to just explain that this fish matches the description of a sailfin and not a bristlenose and that you cannot adequately house it and would like an exchange (ask to speak with a manager or whatever is necessary) and hopefully they'll be reasonable.
Yeah asking for the manager is key...... I don't think you will have too much trouble returning it.