This is one of those epic truth-stranger-than-fiction, nothing-is-too-outlandish Mexican scandals. Ahumada is a convicted felon, and blaming things on Salinas is a trusty fallback for Mexicans under the microscope. In short, he has zero credibility. At the same time, it really doesn't sound all that unlikely; the panistas were desperate for a way to derail AMLO, and Salinas' animus for the Tabascan is well known.
Update: Well, Fernández de Cevallos' response certainly makes you suspect that the allegations are true:
I can tell you that in any case there would be 400 million euros that I owe him or if he prefers 400 million pesos in gold", he said in a joking tone, "I'm not going to enter this controversy, I'm calm, what I did I would do again.
"I've already stated it a thousand times and if he wants for any reason this book to a serve a purpose for him or for third parties, he has the right to do so", the politician assured.