We have a concept of “Zachin l’adom shelo b’fanav”, someone may acquire ownership an item for an individual, if it is to his advantage and even without his knowledge.
When applied to this case of the tzedaka pushke, it seems to me that the intended tzedaka owns the money as soon as it is placed in the pushke, with the understanding that you will eventually forward the cash amount to the tzedaka entity.
But it doesn’t have to be the actual money which was placed in the pushke, in fact, the tzedaka entity prefers that you send them a check instead of the actual cash.
So it follows, I believe, that as long as you eventually give the tzedaka organization its money, you can go on leaving IOU’s in the pushke as a reminder of your obligation.