Fa does not always join two future acts. The textbook explains that when the first element can stand as an enduring cause, fa may explain a causal relation.
«أد إلي ألفا فأنت حر»
Pay me a thousand, and you are free.
In this historical juristic example, the book treats freedom as immediate and the stated amount as an outstanding debt. The demand for payment can remain operative as an obligation even after the legal consequence occurs.
This differs from the earlier circumstantial-waw example: there payment was interpreted as a condition; here the particular causal reading of fa leads to a different result.