"If you are unable to refund the duplicate amount by Friday, February 24, 2017, we will offset this duplicate payment against your future Pond5 sales."
That's what I will do: Not that I need those few dollars now, but they* always pay us at least a month after sales occurred, and more if we don't reach limit. This time it's the other way around.
Playing with cash flow is a wonderful thing, isn't it?
*every agency, not just Pond5