Lets us understand there is no agreement / contract written for donating the leaves.
The crux of the policy is "voluntary donation". Further, in my ex-company where I implemented this policy, a rule was that an employee who has no leaves to his credit, can only seek for donation from another employee/s, of equivalent Grade/level. Hence, leaves could not be donated or asked just at the whims & fancies of people.
I am not sure how u arrived at 18 leaves, but whatever the nos. may be, leaves are credited to the employees leave account without asking. There is no consent taken or required for crediting annual leave to employees account.