Is refraining from paying the rent for the exhibition hall considered Haram (religiously forbidden), knowing that the advertising was not as agreed upon, which led to a decrease in customers?

One must adhere to the terms and agreement of a contract. If one party violates what was agreed upon, the other party has the right to rescind the contract. Your proceeding with the exhibition despite the breach of contract terms is considered your consent and forfeits your right to rescind the contract. However, if you were compelled to hold the exhibition, you have the right to claim compensation for the breach of contract terms and the damages resulting therefrom. This compensation is to be assessed by experts and deducted from the rent. It is not permissible to withhold rent merely due to poor publicity.