First, of course, there is no "totally different chemical composition". The changes are very difficult to detect. In experiments with model animals, the usual method is to insert electrodes into the brain in specific regions and to monitor the electrical activity.
As to your two scenarios, it is the anticipation of reward that is most closely associated with dopaminergic transmission, rather than the consummation of reward. Schultz (2016) describes how dopamine signalling codes for a prediction error using a combination of tonic (slow but continuous) and phasic (fast and pulsed) firing patterns. A pause in tonic firing indicates that an expected reward failed to materialise. A burst of phasic firing indicates that an unexpected reward was received. These excursions from tonic firing give rise to changes in behaviour.
Thus one might expect that on receipt of your telephone calls, there may be some dopaminergic signalling, certainly in Scenario A. In Scenario B things are slightly different because you weren't expecting a reward; you simply received bad news. However, there is evidence (Matsumoto and Hikosaka, 2009) that dopaminergic signalling is involved here, also.
The cells responsible for these changes in dopamine signalling project to the frontal cortex. The receiving cells alter their synaptic connectivity in response. Successful connections are strengthened; connections producing poor results are weakened or eliminated.