Black Stain On Polyester Khakis?

I love shopping at Goodwill for clothes, but a couple of weeks ago, I was in a hurry and grabbed up a pair of polyester/spandex blend (I think, label gone) khaki colored slacks. I just liked the style of them, needed casual dress pants, just my size, long length.

Well apparently whoever owned the slacks last was short because they were safety pinned in instead of hemming. When I unpinned them, to get the extra length, there was a BLACK line around bottom of each leg! I guess it's from the pants still being too long, therefore dragging on ground, pavement, who knows?

I've tried amonia, washed and dried. I've tried dish detergent, washed and dried. I've also tried vinegar to no avail. Stain has gotten lighter in color, but still there! Please tell me find an inexpensive solution to removing this ring! Thx!

CarolinaGirlOnBudget