Is what I am doing forbidden if I buy something under contract terms that I do not accept and I breach them without the seller's knowledge?

Mutual consent between the contracting parties is a condition for a valid sale, based on the Almighty's saying: "Unless it be a trade by mutual consent from you," and the Hadith: "Indeed, sale is by mutual consent." Accordingly, buying or selling under duress without just cause is invalid. This differs from duress with a just cause, such as a ruler compelling a mortgagor to sell the mortgaged item. The recognized form of duress is a threat of death, beating, or imprisonment, carried out by someone capable of executing it. If you were compelled to buy, the transaction is invalid, and you must return the item and reclaim your money once the duress ceases. If the matter did not reach the level of duress, your purchase is valid, and you are obligated to fulfill any permissible conditions. It would have been helpful to mention the details of the duress and the conditions imposed to clarify whether it was true duress or not.