To prevent overdrafts and deficiencies in the fiscal year for which appropriations are made, appropriations made to the Department of Defense or to a military department, and reimbursements thereto, are available for obligation and expenditure only under scheduled rates of obligation, or changes thereto, that have been approved by the Secretary of Defense. This section does not prohibit the Department of Defense from incurring a deficiency that it has been authorized by law to incur.