Certainly, I would agree on all the main points: while unreimbursed expenses incurred in the course of performing services to a qualified organization are tax-deductible to the taxpayer (if they itemize deductions), those expenses are not actually contributions to the charity and the best practice would be that they are not recorded as gifts by the charitable organization.
On the other hand, I would probably emphasize the similarity between the documentation that we provide to the volunteer and the documentation that we would provide in the case of a charitable contribution. The IRS doesn't use the word "receipt" in either case, and in fact uses the same term, "acknowledgment," in both cases.
The acknowledgement provided to a donor who incurred expenses while providing voluntary services must include (from IRS Pub. 526):
These are the same requirements that apply to documenting charitable contributions; the first is very like the requirement to document what the donee received from the donor, and the others are the same exact language. In addition, in both cases the acknowledgment is required when the donor claims a deduction of $250 or more, and the deadline for obtaining the acknowledgment is the same.
The way that I think about why those expenses aren't recordable as a gift by the charity is because it's the services that are provided to the charity, as the acknowledgment says, not the related expenses (and gifts of services of course aren't deductible).
Even so, I think that it's helpful to think of the acknowledgement as otherwise exactly the same as in the case of a charitable contribution. At least, that helps me organize my thinking about what the acknowledgment in this case needs to include!
My US$0.02 worth; the usual disclaimers apply.
Alan S. Hejnal
Data Quality Manager
Smithsonian Institution - Office of Advancement
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