In order to buy back a period of service or a period of absence without pay, you must have an employment relationship with your employer (which must be subject to a plan) during that period.
When you work part-time, your employment relationship exists only for the days included in your work schedule. Therefore, since you do not have an employment relationship during the days not included in your work schedule, you cannot buy back the days during which you do not work.
Jerry holds a part-time job where he works 3 days a week (Mondays, Tuesdays and Wednesdays). Since his relationship with his employer exists only for those 3 days, he cannot buy back the other 2 days (Thursdays and Fridays) because they are not included in his work schedule.
However, if Jerry were absent without pay during the days included in his work schedule (Mondays, Tuesdays or Wednesdays), he would have the right to buy back those days of absence without pay.