How to properly scale-down png assets

I do games for mobiles and web. I have all my artwork in Retina resolution. I use Sketch for slicing my artwork. During slicing I scale-down my artwork for not-Retina screens - and here I face a problem:

Please follow my explanation:

1. 
   

   Here I describe what I do in Sketch app: http://ift.tt/1xeavNg

   
   


2. 
   

   Here I show resulting pictures, describe my problem and ask a question: http://ift.tt/1zIgUq7

   
   

So, my question is how to make edges of scaled-down sliced assets not transparent?

Is it possible to do it in Photoshop or in Sketch?