Volunteer time for my own photography business

None of that is deductible. 

The tax "reduction" is that fact that, by charging less, you have less income to pay tax on.  You can't deduct money from income that was never reported.

Depending on how you want to do your accounting, you could list the service in your accounts at full price and then list a "discount" separately.  That would allow you to use your records to look at the full value of the jobs you do and not just the net payments.  But in the end, you report whatever taxable income you received, and you can't subtract a discount from money that you never included in your income in the first place.