Ma'asar on future earnings
שאלה
Is it possible to set aside מעשר from a wage/paycheck that has already been earned but not yet received (not yet in one’s bank account, למשל)?
תשובה
If already earned and owed to you, the סתימת הפוסקים is that it's fine לכתחילה, as you have in mind to give it as a prepaymemt (if you didn't have in mind, it's more problematic and there are different opinions), as even with מעשר ראשון this would work בדיעבד, so for מעשר כספים we're meikel לכתחילה and aderabe, זריזים מקדימין.
If it's against future earnings, you're basically lending the money to tzedaka. The נוב"י holds it's not ideal, but many, including משנ"ב and others, argue. In that case you should separate when you get and then apply the מעשר to be מוחל the loan to tzedaka.
Note that you can't deduct bad loans from מעשר as you didn't have in mind when lending.