I think the real difficulty lies in the usage of the world "absolute". Now, it is certain that someone like Sartre truly understands words and so his use of 'absolute' cannot be a simple adjective.
It is quite possible that the term absolute means exactly what it means in regular usage and I think viewed from this point of view, Sartre's usage does not necessarily present any difficulties. Having said that, we must maintain what Sartre specifically means by using the word absolute. Absolute, means total. Absolute freedom is all of freedom where freedom is limited by nothing other than itself. That is the definition of absolute.
However, the real crux of the matter for Sartre is not "absolute" but freedom. For what it is this thing which is to be 'absolute'. Hence, it isn't in the world absolute that the problem really hangs rather it is the word 'freedom' which brings about difficulties.
Freedom is an indescribable for Sartre. But just as 'nothingness' is an indescribable and yet Sartre managed to, in some way, explain what it is; he hopes of explaining what freedom. More than an explanation, however, he attempts to describe what freedom is. In the process of describing he de-constructs this concept-like 'Freedom'. Freedom is not a concept, per se, but in order to undertake the task of describing it we must at least initially pretend to treat it as such. Only then will we be able to deal with the intricate relationships that freedom engages in. Freedom is, first and foremost, the nothingness of man - in that sense, freedom is the being of man. The adjective of absolute is derived from this facet of what freedom is. In this sense, freedom is never abandoned, or can never be abandoned because the self can never let of the self. Freedom is the way in which the self makes itself, in the way that it chooses itself, and hence freedom is in that sense synonymous with the self. If one were to, however, take that description of freedom outside of this context and try to posit the self and freedom as equal to each other they would be committing a grave error. The self is not freedom, but it is freedom as long as freedom makes the self, as long as the self intentionally makes itself. Conceptually, or in the realm of concepts, they are not equatable, but once we apprehend freedom subsequent to epoche we can state, in a descriptive language that freedom is self and the self freedom.
Hence, the absoluteness of freedom refers to absolute attachment of freedom to self. That it is full form of freedom which allows the self to make itself. This freedom, which is not manifest from any source outside of the self, and this freedom that resides entirely and fully in the self, with the self, as the self, is the cause and effect of the self, of itself. Freedom is exhausted only by freedom and therefore, the self is exhausted only by itself.
What has been thus far established is only the relation between the self and freedom. Though language presents them as two components conjuncted, the reality is that they are one and the same. Yet, in language we always speak of them differently. What hasn't, however, been developed is what do we exactly mean by 'freedom'.
To be redundant, freedom is absolute. But the absoluteness needs to be in the presence of something. What is that then? For that Sartre brings about the relationship freedom shares with facticity. Now, facticity are all external factors. But as external factors, facticity also includes the individual's past. The self's past, once it has happened acts as external to it. Though we can recall our past, we are not exactly living that moment and as such that past is presented to us in an objective sense. Our past is not necessarily ours in the present moment. Once it has become, it is an object like any other object in the external world. It is a plenum.