Should I return the money to the buyer who purchased a device that works perfectly fine, but is unsuitable for her because of the unavailability of a compatible electrical outlet in her home, knowing that the agreed-upon condition for return was that the device be faulty?

The sale is valid and binding, and you are not obligated to return the device. However, it is recommended to return it since it is not suitable for your sister's house, as this falls under treating people kindly, relieving their distress, and overlooking their faults. This is based on the Prophet's (peace and blessings be upon him) saying: "Whoever overlooks the fault of a Muslim, Allah will overlook his fault," and his saying: "Whoever fulfills the need of his brother, Allah will fulfill his need. And whoever relieves a Muslim of a worldly distress, Allah will relieve him of a distress from the distresses of the Day of Resurrection."