I actually asked a similar question to R’ Shmuel Berenbaum & R’ Dovid Feinstein. My question was, can I do work for a yeshiva & not charge them & count that as Maaser, & can I do work for somebody who I know has no money, & do extra work & not charge them.
R’ Shmuel Berenbaum said absolutely it could be counted as Maaser. He said that you could be Mekadesh a Isha with that value, so why should it not be counted as Maaser.
R’ Dovid Feinstein said that he feels that Maaser is something that has to actually come out of my pocket. He therefore felt that I could not count the time as Maaser unless I charged them & then returned the money