Is it permissible to take the stipulated sum of 5,0 pounds if the doctor fails to pay the value of the ultrasound device by the agreed-upon date?

If what transpired was a sale, then it is not a valid sale due to the unknown price, and the individual's disposal of the device is considered an unauthorized intervention (فضولي). You have the right to demand the return of the device and the annulment of the sale, or to ratify the sale and receive the full price of the device from the unauthorized intervenor, keeping in mind that the trust receipt (وصل الأمانة) is considered void in this case. If the trust receipt was to guarantee payment for the device, then it is not permissible to take the penalty clause amount for late payment.