What bargain was made between Carton and the Spy in A Tale of Two Cities by Charles Dickens?

Carton recognized the spy as Ms. Pross' brother, which would make him English (and by the French standards in the book at that time, a traitor). Knowing that Darnay was in prison at that time, he made a bargain that the spy would take Carton in to visit Darnay before Darnay could be executed, in return, Carton would not reveal the identity of the spy (which would result in the spy's death if his identity were discovered).
Carton promised he would not reveal the spy's identity to the revolutionary government