As I understand your problem you are attempting to replace the down tube from your S trap where it goes into the house plumbing at the floor. Since it is a sink drain I will assume you have a 1 1/4" drain going into 1 1/2" house plumbing and someone helped you out by soldering the two together. First question: Can you tell what material the the house drain is made of? IE: galvanized, copper, lead? Second question: Are there any threads left on the end of the pipe coming through the floor? If there are usable threads you can get a thread adapter and try to screw it onto the pipe and use a compression drain fitting. If the pipe is copper, you can sweat a thread adapter onto the end of the pipe and again go to a compression fitting. In either case, you should have no damage if you heat the pipe and remove the existing solder. If it is lead you will have to be extremely careful when removing the solder not to damage the house drain. That might even be a job you want to find a plumber for depending on your experience and comfort level. I am assuming ( you know what happens then ) it is not plastic if it was soldered before. Let us know how it goes.