My father’s yurzheit is coming up on a yom tov (and there is competitive bidding) and I reserve money to possibly purchase an aliyah. But someone called me in desperate need of Tzedekah in the amount I would have used for the aliyah, can I give this poor person the money I would have given to purchase that aliyah, and have in mind l’zechur nishmas my father, a’h?
It is better to give the money for tzedakah l’illuy nishmas your father. The mitzva of tzedakah is a great zechus for his neshoma. As an added thing, after giving the money to tzedakah, you might want to consider buying hagba’ah, which will be a lot cheaper, and it is also an zechus, (see M:B 147-7).