Why did Allah forbid David (peace be upon him) from breaking his oath ("Take in your hand a bundle of rushes"), while He commanded Prophet Muhammad (peace and blessings be upon him) to break his oath and offer expiation for it ("Allah has already ordained for you [Muslims] the dissolution of your oaths")?

The one whom Allah commanded to fulfill his oath and not break it was Ayyub (peace be upon him). He had sworn to flog his wife one hundred lashes, so Allah commanded him to take a "dhighth" (a handful of twigs) and strike her with it, so that he would fulfill his oath and not break it. Scholars interpreted the phrase "and do not break it" in one of two ways: either because there was no expiation for oaths in their law, or because what he uttered was a vow and not an oath. As for Allah's saying to Muhammad (peace be upon him): "Allah has already ordained for you [Muslims] the dissolution of your oaths," its meaning is to clarify the permissibility of breaking an oath and its expiation, which is feeding ten poor people, as stated in verse 89 of Surat Al-Ma'idah. Therefore, the Prophet (peace be upon him) would expiate his oath if he saw something better than it.