Warning, non legal advice follows:
If they offered you a specific salary and expenses at interview, even verbally, and you accepted and agreed to those conditions, then you have entered a contract. If they then choose to change these details it's up to you to challenge it.
You need to weigh up the pros and cons of this, the worst case being the job offer cancelled. However as it is an agency, there might be more scope for negotiation, as whatever they are paying you, they will receive far more from the airline.