Maaser ani: Designating and giving ma'aser ani in advance
Question
I have a fruit tree in my yard. I'm going to see a friend today who lives far away and that I only see occasionally, and that friend is any position where they need ma'aser ani. Can I pick fruit to give as ma'aser ani now, in advance, before I pick all of the fruit that that ma'aser ani belongs to?
Answer
While it is technically possible to separate terumot and ma'aserot in advance (see my explanation here, it is halachically problematic to designate and give ma'aser ani ahead of time. The reason is that the fruit must still exist (and not be consumed) at the time of your full harvest, when the rest of the ma'aserot are separated.
A practical alternative is to give your friend the fruit as a substitute for ma'aser kesafim, tithes from monetary income. You can deduct the value of the fruit from the amount you are obligated to give from your income, and this in-kind contribution certainly qualifies for ma'aser kesafim.