If you already gave a tenth to maaser for this year and you realize you gave more than you had to, are you allowed to apply it to the coming year’s maaser?
Yes you can apply it for the coming year, but you should just say that what you gave then is really being given now. The reason for this that since the extra money that you gave was by mistake, the halacha is that maaser that was given by mistake can technically be taken back. Therefore by saying that you want it now to go for the maaser of this year, that would be sufficient.
R’ Leshinshy Shit”a