That sounds like a pretty nasty scam. Pushing quickly for the conclusion, then really demanding high cancellation fees.
That's how you generate revenue without having to do anything.
Without knowing your contract, no one can help you here. But you have to take responsibility for not having negotiated a cancellation right. With this amount, without owning a property, without having the financing secured, it was more than careless/naive.
It is signed, and the contract is legally binding if no mistakes have been made, or if you can't prove that you were "taken for a ride."
We also don't know exactly what was not delivered. If a serious deception really took place, I would, in your position, possibly even pay the 15% and be glad not to have lost even more money. But ultimately you can only try to negotiate your way out of the contract. Have you ever asked if the provider is willing to declare the contract void? (You always clarify such things with the boss.) Explain your position to him, and try to keep your composure during the conversation. Do not become insulting, etc....
If they refuse or don't offer you a lower percentage rate, then only a lawyer can help you.