Deductions & credits

Sorry for the confusion. Essentially I am signed up for an FSA for 2018 and 2019. In 2018 I spent $500 on treatment. In 2019 (late in 2019) I got a full refund for the treatment. Part of the refund included the $500 from 2018. 

so now the question is what to do with the $500 from 2018. I believe I can send it back to the FSA admins, use it on medical expenses in 2019 (which is the easiest way), or report it on my tax return (?). 


since the IRS allows for a $500 carry over from the previous year, can this money just be used for 2019 Medical expenses? 

Any thoughts are appreciated. Thanks.