Owwiee... I would do nothing at this point. Most times it will either heal or fall off
You can wash it and apply an topical ointment that inhibits infection but I never do this... seems it just causes the bird to pick at it more. I have never had one get infected yet but if I did notice redness and swelling I would put the bird in a cage and administer antibiotics to the drinking water and wash and dress the toe.
These finches are much tougher than you would think.
I would check your cages to see where this toe mishap could of happens so the you can fix the problem to prevent others from getting injured. Cage corners, wicker baskets and rope perches/swings are all places that can trap toes.