Halogen-to-boron π donation reduces electron deficiency at boron. Strong 2p–2p donation from fluorine makes fluorine-rich boranes less Lewis acidic; replacing F with heavier halogens generally weakens this donation. Within the difluoro pair, the bromide is stronger than the chloride.
Thus the intended order is **BFBr2>BFClBr>BF2Br>BF2Cl, B**. This is the usual comparative Lewis-acidity trend for these molecules.