Is withholding an employee's full wage—after he failed to completely fix the defect, but incidentally fixed part of it—considered a usurpation of his right, and a violation of the saying of the Messenger of Allah, peace and blessings be upon him: "Give the employee his due before his sweat dries"?

Repairing devices falls under the category of ju'alah (reward-based contract). If the worker cancels the contract after beginning the work, he is not entitled to anything, because he forfeited the completion of the condition himself. However, if the cancellation is from the ja'il (the one offering the reward), then he is obligated to pay the worker the ujrat al-mithl (customary wage) for the work that has been completed. As for any work done after the cancellation, there is no wage for it. Therefore, if the worker does not return to complete his work, he is not entitled to anything.