In cases of a member’s willful substance abuse or if the board determines a member who qualifies for disability retirement under this part pursuant to this chapter has mental, physical, or vocational rehabilitation potential, the board may limit the disability retirement to a period not to exceed two years from the date of approval of the disability retirement. Notwithstanding Section 24112, the disability retirement allowance shall terminate at the end of the period granted unless an extension is granted by the board.