That gash makes me curious what happened? I would probably either remove the entire cuff (as it looks sewn on to the rest?) and replace it with something else - fabric (denim?) or leather if you can get hold of some. Or I would add something to the back - fabric in a similar colour - and use leatherwork needles to sew the gash shut with x-stitching. That way you close the gap, keep the leather look and get a “feature” from the stitch.