I am not a Lawyer and cannot give Legal Advice,
but would suggest that All Parties
(Buyer, Seller, Lender, Real Estate Agents, etc.)
SIGN an acknowledgment that it is a One-Car Garage,
and not a Two-Car Garage as stated.
Even though it should be obvious to anyone looking at the garage,
this may be mis-representation.
(Where was it stated?
In an advertisement, in the MLS?
Is a Listing Real Estate Agent involved?)
Again, get all parties to acknowledge that it is a One-Car Garage.
GET A LAWYER if necessary.
As far as "Selling it Later", check Market Statistics for the neighborhood and the area.
What do other similar comparable townhouses have?
Do two-car Garage townhouses sell for more money?
Do they sell faster or "easier"?
Get the local market data.
A licensed certified registered Appraiser may be able to advise.