Is a master's degree considered an effacer of previous cheating in lower degrees, knowing that the master's degree is what was presented for employment, and is the certificate considered incomplete due to the previous cheating?

If the employee is competent and performs his work proficiently, then his salary is permissible (halal (definition)), and his previous cheating does not harm him. Sheikh Ibn Baz issued a fatwa (definition) that the job is valid and the salary is permissible as long as the employee performs his duties, and he must repent from cheating. Ibn 'Uthaymeen mentioned that the decisive factor is the final certificate. So, if he cheated in previous levels but did not cheat in the final one, that suffices.