I don't really care if module B gets bad input from module A, as that is most likely taken care of as a generic input error. But I do care about the correct values returned after I invoke Service A, which I then need to send to Service B. Service B can only be invoked after I get some values from Service A.

Mocking Service A sounds like a good idea, and I may look further into this, provided the sandbox returns the same results everytime