This situation is indirectly discussed in IRS Revenue Proc 2004-43 at https://www.irs.gov/pub/irs-drop/rr-03-43.pdf .
On page 3 in the paragraph starting with "Under Employer N’s card arrangement, a few of the claims that have been reimbursed are subsequently identified as not qualifying for reimbursement" the paragraph suggests that if you received funds that turned out not to be for a qualified medical expense, then you owe the money back to the employer. Thus, you would need to turn the $300 over to your employer. This would have no tax consequence for you. Why? Because if your employer had not paid the money in 2016, the amount would have been forfeited at the end of the year as unused (yes, I am aware of the 2 1/2 month grace period and the $500 carryover in some plans - let's don't make this more complicated than it already is).
Elsewhere, the revenue proc suggests that FSA payments which are not for valid medical expenses must be added back to the employee's income. However, in this case, the reimbursement of the funds took place three years later, which probably makes it difficult to update paperwork properly both for you and your employer. For example, it wouldn't make sense to amend your return for 2016 to declare this as income when you didn't even receive the money until 2019.
For this reason, I would agree with the previous comment that you should go your employer's HR department and offer to pay the money back. If by some chance you can't go back to your employer, then cash the check and declare it as Other Income on your 2019 return. In any case, you should avoid receiving the $300 AND getting a tax benefit for it.
