According to Theravada Buddhism, nirvana doesn't escape the causal process, i.e., dependent origination. Dependent origination even extends to the absolute. The causal process has no outside, in other words there's no escape. Thus, nirvana is in connected with conditionality. We get a flavor is this when the Theravadin monk/scholar Walpola Rahula tries to force this passage, “All conditioned things are impermanent” (sabbe samkhâra aniccâ) to include nirvana which is not conditioned; which is outside of the causal process. He writes the following in this book, What the Buddha Taught.
“The term dhamma is much wider than samkhâra. There is no term in Buddhist terminology wider than dhamma. It includes not only the conditioned things and states, but also the non-conditioned, the Absolute, Nirvana.” (p. 58).
From the perspective of the Theravadin insistence that dharma includes even the conditioned assumes that the word dhamma/dharma has only one particular meaning in the Buddha’s discourses when, in fact, it has many meanings. Dharma can mean: 1) The teaching of the Buddha; 2) Behaviors including practices; 3) The absolute truth when one sees it with dhamma vision; 4) The defining feature of something; 5) The foundation of reality; 6) Mental and physical things/qualities. These six, less that adequate meanings of dharma, are by no means the whole enchilada.
If nirvana is a dharma, for the sake of argument, it is certainly not one that has mental or physical qualities nor is it text or a practice. In the Udana nirvana is described this way:
“Monks, there is a not-born, a not-become, a not-made, a not-compounded. Monks, if that unborn, not-become, not-made, not-compounded were not, there would be apparent no escape from this here that is born, become, made, compounded. But since, monks, there is an unborn ... therefore the escape from this here is born, become ... is apparent” (80).
Here the Buddha talks about an escape from mental and physical phenomena which are born, become, made, etc. which are otherwise known as the Five Aggregates or the psycho-physical body. This escape is nirvana or vimutti. If Theravadins are trying to make us believe conditioned things and the unconditioned, that is, nirvana, are the same as far as dharma goes, they haven’t succeeded.