2 days ago I did finally cross into Guatemala, at the Frontera Corozal border crossing, after I visited the Yachilan archeological site. Then I took a boat across the river and entered Guatemala.
I paid the 500 pesos, which seemed like a routine matter to the Mexican INM official. I´m not at this point sure if I could have avoided paying 500 pesos by attempting to take care of the matter within 7 days. The immigration officials in Ciudad Juarez in fact told me that I´d need to pay $500 when exiting the country, at the time they stamped my passport. What I failed to grasp was that the $500 pesos was connected to the 7 days they allowed me on my FMM permit.