Yes, you may pay your Zakah early. However, you must make sure to pay the entire amount you would have owed on your due date. For example, if you predict you will have $10,000 on your Zakah due date then you would pay $250 in Zakah. If you prepaid $100 a month earlier, you would now only have $9,900 on your Zakah due date. You must add the $100 back to $9,900 in order to calculate that you should have had $10,000, so you now owe the remaining $150.
Shaykh Mustafa Umar