Can one reduce his monetary tzedoko by work that he donates to a charity?
For example, someone was employed (for payment) by a charitable organisation. Then the organisation could no longer afford to pay his wages. He then volunteered to give his work for no payment.
Can he reduce the amount of monetary tzedoko that he gives by the wages he used to get?
(If it helps answer the question, we could assume that he will not reduce his monetary tzedoko below 10%.)
Obviously a practical case requires CYLOR but what sources can be applied to the question?