Don't return the sailfin. More harm is usually caused in stressing him out by recatching him. (I'm sure others will disagree with me here)
I would say, that it is about a 50/50 with whether or not he will make it. I guess that is all part of buying tangs. I don't know if I can offer any good advice. Sometimes the yellows give in, and accept the sailfins, sometimes the sailfins can't handle the stress. I guess the main thing I would strive for is to make sure the sailfin is getting plenty of food.
just my thoughts, and I hope they help,
Adam