Is it permissible to hasten breaking the fast in Ramadan half an hour before the Maghrib call to prayer, on the pretext of observing the sunset, and is there any harm in that?

The meaning of "expediting the breaking of the fast" is to break the fast immediately upon the complete disappearance of the sun's disc. The time for Maghrib begins with the sunset and its complete setting, which occurs when the disc fully disappears. The lingering rays are not to be considered. If there are mountains, the consideration is that the sun's rays should not be visible on the walls, and darkness should be approaching from the East. Relying on someone else's report of sunset is permissible if they are in a nearby location; however, if they are far apart, it is not permissible, as the time of sunset may differ. If there is doubt about the sunset, it is not permissible to break the fast until one is certain or has a strong presumption that the sun has set.