1500mA is pretty huge, it was a typo in the first versions of the User's Manual.
Consider that an USB slot should theoretically deliver up to 500mA (much less in most cases), so 500mA is more than sufficient for your keyboard.
Basically, you won't damage your unit with a power supply that delivers more; you must consider that the keyboard "asks" for some current, the power supply "delivers" this current until the maximum it can provide is reached.