Since you're only going to use it for the summer, just get this Bridgeway from REI and cut your losses:

www.rei.com/product/855378/marin-bridgeway-bike-2012-overstock

I'm sure you could resell it on CL for at least $300

There's also the KHS Alite 40, a rigid fork 26er mtb:
http://khsbicycles.com/bikes/2013-khs-models/alite-40/

I'm sure you could advertise this bike, as a brand new mtb, for only $300.
Have the bike shop dealer swap the tires for some slicks, and use the slicks as a selling point, after the summer.