Dear Sir, according to the principle of freedom of contract, an agreement may be terminated at any time by mutual consent of the parties. In this consent, the parties shall determine in particular the consequences and the mode of termination the contract. In such a case the contract is terminated on the date agreed by the parties in the consent. If the mutual consent is not reached, the breaching party is liable under Art. 471 and cons. of the Polish Civil Code. It means e.g. that You may incur the liability for damages raised on the other contracting party of the agreement as the consequences of the breach of contract, on the other hand You may be obliged to incur contractual penalty which shall be specified in the agreement. I wish You the Best!