It would appear your using a commercially made holster that doesn't have a sight channel formed into it. A properly formed holster will be wet formed to allow clearance about half way down, on a revolver holster, to accommodate the front site. You may be able to case the holster, depending on the weight of the leather. Add a dowel the height of the front sight tapering back to the height at the front of the frame, tape it in place, wrap the revolver in saran wrap and fit the gun back in the holster, finger press the shape in, and let it dry which will form a sight channel, and over all tighten the holster up again with a nice fit. Holster makers use blue guns to plan out a new designs and then just stamp them out over and over. On revolver holsters they do minimal fitting if any.