How is it a scam, why didn't you screenshot the post where you commented that you thought the white paper was good? Oh an why didn't you tell the members that you didn't pay for it?
He/she probably won't have payed you if they didn't like your designs/writings. Unprofessional works don't need to be payed each time.
And if he can't screenshot the post/comment where your work was appreciated than why don't you screenshot it for us?