Define a function in your fileTransfer function that you call with system.util.invokeLater.
Example:
import shutil
label = event.source.parent.getComponent("Label")
def fileTransfer():
path = system.file.openFile()
newPath = "F:\NewFolder"
shutil.copy2(path, newPath)
def func():
label.text = "Step one done"
system.util.invokeLater(func)
system.util.invokeAsynchronous(fileTransfer)
Functions called with invokeLater are always run on the GUI thread so this is safe.
Best,