The best advice I have seen is to use a ice cube near its foot. Just rub it a little and he should start to let go. If its on glass, you can (blow dryer or similar) use a heat source to heat the spot on the glass to get it to become 'uncomfortable'. I wish more pet/
lfs's realized what they are doing when the snatch the poor thing out of the holding tank when someone buys one. No wonder we suffer so many losses..
Peace.