I had provided $600 to one of my friend in US and is green card holder. He returned that money back to me in two weeks. Do I have to mention it in SF-86 form ? Please kindly let me know.
I think you answered your own question here. You provided financial support to a foreign national. Green card holders are not US citizens, therefore they are foreign nationals.
List it on the form. You’ll need to list the details including what foreign citizenship the person holds.
You loaned money to a non-citizen. List it and explain it. It’s that simple. It shouldn’t cause you any problems unless you knew that he was going to buy drugs with it.