Suppose I know I'm going to be paid a certain amount, but the check hasn't yet been delivered.
Can I give a Maaser from it before I get the money?
For example to a tzedaka's Chinese auction/raffle that happens before I get paid (from what I've seen online you can give Maaser towards a raffle ticket).
Yes you can donate money now and deduct it from future ma'aser "obligations". I found it clearly adresed by dinonline
The Noda Beyehuda [volume 1 siman 73] rules that one may give preemptive maaser on money he has not yet earned and deduct it from later earnings. This is quoted in Pischei Tshuva [Y:D 249]. This is also the opinion of the Chasam Sofer in his chiddushim to Maseches Gittin 31b.
PS. Congratulations on asking this question. R Moshe Feinstein was complaining he was getting too few questions on how to properly discharge tsedaka obligations !
To elaborate Mbloch's answer a bit:
The Maaser Ksafim has two parts: the monetary part and the Mitzvah part, which is not exactly equal to Tzedakkah (as many confuse).
The Mitzvah is learned indirectly from Yaakov's "וְכֹל אֲשֶׁר תִּתֶּן־לִי עַשֵּׂר אֲעַשְּׂרֶנּוּ לָךְ׃", and is [partially] Midat Habitachon and gratitude (not equality of support as in Tzedakah). The later cannot be enacted before receiving the money, just as the Posuk says - receive first and give later.
Therefore, even if one can be Yotze the monetary part and the most of the second part (as Mbloch answered), the best way of performing the Mitzvah is after you get the money. To the best of my knowledge, it might be the difference between Lechatchilah and Bedieved. So Lechatchilah the Maaser should follow the money.
Source: personal talks with R' Shlomo Shlezinger Z"L.
