In Chapter 7, Achebe foreshadows an event that will become one of the factors in Obi's eventual decision to accept bribes:
But six months might be long enough for things to improve a little. No one told him about income tax. That was to come, but not for another two months.
The phrase “that was to come” is a direct instance of foreshadowing: Achebe explicitly declares an event that will occur later in the novel. This instance is one of a range of moments when Achebe reminds readers of Obi’s broader trajectory and his slide into corruption—starting with the beginning of the novel during Obi’s trial, and ending with his arrest at the novel’s conclusion.
This is also an example of dramatic irony: from this point forward, the reader knows that Obi will be caught off guard by an income tax, but Obi himself does not. This contrast instills foreboding in the reader. It is already clear that Obi is struggling to keep up financially; the knowledge of the looming income tax makes readers guess what will eventually drive Obi to accept bribes.
This passage also accentuates the difference between formal education and knowing the ways of life in Lagos. Despite his education and intelligence, Obi doesn’t know to expect an income tax. He is ignorant of the systems he lives within, which are systems of colonialism: no amount of study can prepare him for the realities and injustices of life in colonial Lagos.