Mouse Up Event

HTML

<script src="https://rawgithub.com/kangax/fabric.js/master/dist/fabric.js"></script>
<canvas id="c" width="600" height="600"></canvas>

CSS

canvas {
    border: 1px solid #999;
}

JavaScript

var canvas = this.__canvas = new fabric.Canvas('c');

var rect = new fabric.Rect({
  left: 100,
  top: 100,
  fill: 'red',
  width: 100,
  height: 100
});

// "add" rectangle onto canvas
canvas.add(rect);

canvas.on('object:scaling', function(){
    var obj = canvas.getActiveObject();
    obj.set({
        opacity: 0.5
    });
    canvas.renderAll();
});

canvas.on('mouse:up', function(){
    var obj = canvas.getActiveObject();
    obj.set({
        opacity: 1
    });
    canvas.renderAll();
});