Not sure what to say, why work with a gunky dirty and currently non-fnctional used part? Easy eliminate a possible failed part, a USB port is probably under $1 if you buy a bunch on tape, but I am seeing around $5 for one on a quick ebay search try: microusb port and sm and find one with the right anchor pad tabs to match that of a N900.
learn how to solder, very good video https://www.youtube.com/watch?v=jGZKsuaJlpo
and read this thread
http://talk.maemo.org/showthread.php?t=75920
most people won't try this fix, maybe you will and will suceed. WHat do you have t loose? A blister on your finger?
FWIW I have done the N900 USB fix maybe three times with only a roll of 0.7mm 50/50 electronic rosin flux solder a desk lamp a 15x eye loupe and a terrible cheap Frankenstein soldering iron made form the heat coil of a 220v iron and a slightly better tip but still junk 110v US soldering iron.