Am I entitled to cunningly take back my right from the seller after the device I bought from him broke down and he refused to repair it or return its price, knowing that I had informed him of problems with the device since its purchase, but he paid no heed?

If the defect in the device is due to a manufacturing flaw that existed before possession, then you have the right to return the device and recover the full price, or to keep it and receive compensation for the defect. However, if the malfunction occurs after possession, you do not have the right to return it. In case of dispute, the matter should be referred to a judge or mutually agreed-upon experts. The issue of "al-Dhafr" (seizing one's right by stratagem) is a matter of scholarly disagreement.