I am currently doing my first deposit bonus and I am half way there. I withdrew my $600 and the bonus is fine. Here is what a support staff said to me:
" Hello Jordan, Thank you for your response. At this time I can confirm that you have made a deposit of $600 for your first time deposit bonus. You have already earned close to half of this bonus. Feel free to do any cashout as it will not affect the release of the remaining bonus. You may check the status of your bonus at any time from the cashier window in the lobby. Please let us know if there is anything else that we can do for you. Regards, Sebastian P PokerStars Support Team "
However this is only because I deposited the full amount. If for example, you deposited $300 for the first deposit bonus and cashed out, you will still get $300 bonus money but you cannot make another $300 deposit to get the remainder $300 bonus. So I strongly recommend you deposit the full amount before making any cashouts.
Hope this helps.