As nouns the difference between dispensation and dispensability
is that dispensation
is the act of dispensing or dealing out; distribution; often used of the distribution of good and evil by god to man, or more generically, of the acts and modes of his administration while dispensability
is (uncountable) the state of being dispensable.
As an adjective dispensable
is able to be done without; able to be expended; easily replaced.
As a noun dispensability is
(uncountable) the state of being dispensable.