"Pagar" is a tricky verb because it can take a direct object, an indirect object or both. It can even take two indirect objects. It has puzzled me in the past:
Even native speakers seem to stumble over it a bit:
But, one thing that Anderlc says in that discussion is relevant here:
What I can surely tell you is that, when you're using "pagar" with the meaning of "paying someone", you must use the preposition "a".
That's what I think it is here.