I agree with this concern - one of the most common complaints I hear from my friends and coworkers is that they keep getting extra responsibility handed to them, but never actually see the promotion. There is an incredibly delicate balance between motivating people to take on new challenges and discouraging people by not giving them enough in return for it.

The best way to handle this situation (for the employee) is to just sidestep the problem and offer the increased salary and promotion upfront with the new responsibility. There is the additional benefit of internal visibility - if somebody is taking on extra responsibility, it is able to be communicated to the whole team, dodging the common issue that engineers face of not knowing who in their organization they should go to for help/mentorship.