May I take money out of a pushka and replace it with a check? (Essentially cashing a check).
Keep in mind that often the organizations even ask the donor to send them a check and keep the cash and coins.
No, you cannot cash a check with the pushka. This is a service that some take money for, and one cannot use the pushka for it.
Although one can change coins with a pushka, because this involves no loss, a check involves possible inconvenience, and the loss of money for at least a few hours.
Therefore, before changing a check with coins from a pushka one should ask the gabbai if he doesn’t mind. If he gives his OK, there is of course no problem.