You don't "pump power" into a load. From [u]Ohm's Law[/u], Current = Voltage / Resistance.
900mA = 0.9 Amps is the limit of your USB port. If you draw more current than that, it will shut down,or the voltage will drop, or it will fry. (A USB port will usually just shut down, but I wouldn't intentionally try stressing it.)
We don't generally don't know the resistance/impedance of the Arduino or Raspberry Pi, but we could measure the current. The current will depend on what's attached... i.e. If you drive a bunch of LEDs with the Arduino, it will "draw" more current. Sorry, I don't know how much current it draws with nothing attached. The Internet tells me the Raspberry Pi draws a little more than 300mA with nothing attached.