Someone paid a bill for me which I could not pay – do I have to give maaser on the amount that was paid.
According to most opinions, because you did not receive the money, there is no obligation of maaser.
The Orach Tzedakah (p. 370) writes that according to Rav Nissim Karelitz, one does not have to take maaser from rent money that was paid by parents. This follows the majority opinion that maintains that there is no obligation to give maaser from gifts.
According to some opinions, however (as quoted there) there is an obligation to given maaser from gifts, and the same will apply to money given for a specific purpose, or to money that is used to pay a debt.