Reuse non-extended h1b after 2 years of working outside of usa

Hello,

I am in USA on H1B from April 2017. My current H1B duration expires on September 10th 2019. I need to work from outside of USA for 2 years from now, also have to leave my current employer to do so.

So, my question is : If I do not get my H1B extended now but leave USA, work outside for 2 years. Can I use my H1B to come back to USA after completion of those 2 years? i.e. somewhere in 2021/2022?

Yes, you can use that. You can file as Cap Exempt and come back. You could apply for H1B transfer and come back.