Disclaimer: This is my opinion, one may disagree with me.
1. You may not take money out of the box, that money is not yours once it goes in.
2. You may change your mind as to where the money will go once it is in the box. But it may not go to you, it is not yours anymore.
3. From now on, you may make a t’nai when putting money in the box. Say that this money is put away for any purpose that I so choose. It is then not tzedakah money yet. When you feel like giving the money, then give it to the tzedakah of your choice. If you need to “borrow” from it, you may. You may then choose to replace that borrowed money (it is yours and not really borrowed) or even add to it, that is up to you.
4. You make make a general t’nai that from now on, you may borrow or otherwise use the money in the box until it is given to tzedakah, and that t’nai will remain in effect until you annul it.
I would recommend renewing the t’nai every Rosh Hashanah.