What conditions must be met in order for voluntarily forgone income to count as tzedakah?

Question

Suppose one has done work of some value for a charitable organization in exchange for money. This organization would normally qualify as a destination for tzedakah, and perhaps you have also given this organization money in the past. Now one of the following situations arises:

1. You tell the organization to keep the money as your donation to them, since this will lower your marginal tax rate (in the U.S., this donation would not count as income, but may bump you up to a higher income tax bracket, which would increase your tax liability on income from other sources).
2. The organization asks to keep the money as your donation to them, since this will reduce their payroll tax obligation, and you consent.
3. You agree ex ante (lechatchila) to accept a lower hourly wage than what you charge other non-tzedakah organizations for identical work, with the difference being an effective donation.
4. You agree ex ante to accept a lower hourly wage than what your peers are charging this same organization, because you are more wealthy than them and can afford to give the organization a larger donation of your time and money. However, you do not do similar work for non-tzedakah organizations, so there is no true reference for how much the work is "worth."

In which, if any, of these situations can you count the forgone income towards your ma'aser kesafim obligation?

Note the two situations above may or may not differ in that 1-2 are ex post (b'diavad) agreements, and 3-4 are ex ante (lechatchila).

Answer 1

What I heard from Rabbi Yitzchak Breitowitz is as follows:

If I provide a charity with $100 worth of services (let's assume that the standard going rate, had they paid for these services, is $100), I can take $90 off my maaser kesafim (tithing of funds). Had I done the work for-fee, I would have made $100, of which $10 would have gone towards tzedaka and $90 could have gone in my pocket. Hence, as I did the work for free, that's $90 less in my pocket, so write off $90 from maaser kesafim.

Based on this, I don't see the distinction between 1&2 and 3&4.

But it's a good question, what if my services would be priced very differently from the going services.

If it's legitimately $100 worth of lawn-mowing (or whatever service), then it makes no difference whether we agreed "I'll take $100" then I decline to take it; "I'll do it for $50" then I decline to take the $50; or "I'll do it for free."

If you receive some compensation but less than what the service would normally cost (e.g. you do a $100 lawn-mowing job for $50), once again, you can take 90% of that difference ($45) and write it off to maaser kesafim.