Sabbatical leaves may be granted for periods of one year at rates not to exceed one-half basic annual salary, or for periods of one-half year at rates not to exceed full basic annual salary. Eligible employees on sabbatical leave may, with the prior approval of the chief administrative officer, accept fellowships, grant-in-aid, or earned income to assist in accomplishing the purposes of their leaves. In such cases, the chief administrative officer may adjust the sabbatical leave salaries to reflect such income, either prior to or during the periods of such leaves; provided, however, that in no case shall sabbatical leave salary be reduced if total earnings are less than full salary.