if one pledged money to a shul for a kibud and then at a later date earned money, could one give the maaser money for that pledge?
Answer:
Once the aliyah has been pledged and “purchased,” one cannot use maaser money for paying it, unless one had in mind at the time of the pledge that maaser money will be used.
The reason for this is that after the event, the pledge is a monetary obligation, and maaser money cannot be used to pay it off.
Sources:
See Taz, Yoreh De’ah 249:1; see also here, letters ches-tes.
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