Legally, they are on solid ground. From a business and marketing POV it's a real mess. Any good sales manager would either let you chop the bike back in at full retail or maybe do it - a 10% restock fee or similar.
I would suggest that you state your intentions to them. That you are happy to let them hold the purchase price until the new bikes come in.
If they aren't happy with that then they really aren't a good shop to be dealing with.