Is it permissible to refuse to pay the extra repair cost for a device, due to a lack of prior agreement on it?

If the company repaired something in your device that you did not agree upon, then you are not obligated to pay for it. This is because the default principle is that whoever performs work for another without an agreement is considered a volunteer, unless the person is accustomed to charging for such work (like a cupper or a tailor) and was given permission, in which case they are entitled to a "fair wage" (ujrat al-mithl).

Two cases are exempt from this: 1. Rescuing another's property from destruction (such as saving belongings from the sea or from the mouth of a wild animal). In this case, the worker is entitled to a fair wage, even if the owner did not give permission, as an encouragement to save property. 2. Returning a runaway slave (according to the Hanbalis) or a stray animal (according to the Malikis, from those who are accustomed to returning them for compensation).

Therefore, if the company repaired something additional without an agreement, they are not entitled to payment, and they should have informed you beforehand. However, if the repair was beneficial to your device (especially if the device's functionality depended on it or if it was customary to repair such faults without detailed consultation), then you should pay them for that.