I have a couple of statements in my T&C's to cover this type of occurance. Something like this:
The client's art director is to give a "shot list" with some sort of expectation for resulting images.
If no art director is involved, than I will shoot my interpretation of their direction, using photographic composition determined by the photographer.
If the customer demands a re-shoot due to a lack of direction on their part, customer pays for the re-shoot.
It may sound a bit detailed, but it addresses exactly this type of problem. I feel the contract should indicate/manage the work to be done, the deliverable, the price and who is responsible for what and what each party is required to provide.
If this is the 1st time you're shooting for this client, chances are they're just looking for an excuse to "stick" you. If so, you probably don't want to work for them in the future.
When I deviate from my price sheet, I always quote the full price in my contract but indicate I'm giving a discount. Call the discount "introductory" if you want, but it set's the tone for the next contract and your pricing.