The setting for this one is the U.S. in the 20th century or possibly early 21st century (as imagined from a 1970s perspective). That is to say, it's modern day or near-future from a technological perspective. The protagonist (a truck driver, IIRC) has learned that someone has reported him for not paying his taxes a few years ago. In this U.S., that is a capital crime, and the sentence is delivered via an IRS assassin, with no appeal or clemency possible.
I am a bit fuzzy on how the assassin first contacts the protagonist. I think he pays a courtesy call to the man's house. I seem to remember something about a legally-required period of notification, and an offer to make it quick if the man surrenders, but I'm very iffy on those points. What I do recall is that protagonist flees his home, with the hopes of reaching Mexico (and thus, escape his death sentence).
The story proceeds with a cat and mouse type pursuit with several close calls. As the man nears Mexico, he sees his final obstacle to freedom. The U.S. has built a massive wall along the border, to keep people from escaping the U.S. (IIRC, there are other reasons besides the tax one) The man crashes the vehicle he is driving (with the IRS agent - assassin close behind), but manages to get over the wall.
The man thinks he is safe, but then he learns there's a trick. This is actually not the border wall. That's several hundreds yards further south. The "dummy wall" was put there to trap people trying to escape. The IRS agent explains all of this to the man as he prepares to shoot him dead. The protagonist is about to give up all hope, when he remembers something. He quickly asks the agent about the year he didn't pay his taxes. He then points out that he made too small a sum that year to have owed taxes in the first place. The agent reluctantly admits that the man is right, and leaves the man with his life.
The story closes with the protagonist walking towards the true border wall, secure with the knowledge he is a free man.
Time or Era of Publication
I'm almost positive I read this one in a sci-fi collection from the 1970s. It could be older than that, but I don't think it was from anything more recent than the 1970s.