The building did not become darker in the second picture, the colors are different but the brightness is about the same.

There are two differences between the photos:

1. White balance.

   The building is lit with somewhat yellow lights, in the 1st picture the camera compensates and makes the light white.

   For the second picture the flash's light is blue, the camera again compensate making it white - but that makes the yellow light of the building even yellower.

   If you just set the white balance for the building the girl in the background will become blue (I guess you don't want that) - to get teh right color for both foreground and background you need the color of the lights to match, you do that by using a "CTO gel" (basically, an orange transparent film) - that will make your flash also yellow and let you get both the building and the girl in natural color.

   Note that it probably won't work with auto white balance.

2. The building is out of focus

   Since it is dark outside you are probably using a large aperture to get more light into the camera, light aperture causes shallow DOF and so you can't get both foreground and background in focus at the same time.

   To get more in focus you will need to close down your aperture (use a higher F number), this will make the exposure longer, it won't effect thee person in the foreground much because the flash's burst of light is still very short but you will need to hold the camera steady longer to avoid motion blur in the building - this can be solved by using a tripod or placing the camera on some solid surface nearby.