The rules for noncash charitable contributions defy easy summary. On the other hand, they are not rocket surgery. Moving on from the humor, if you want to sum them up in a sentence you can use Reilly’s Seventh Law of Tax Planning: Read the instructions. Specifically, you want to read the instructions to Form 8283 Noncash Charitable Contributions.
There is, of course, more to it than that, but you will find a remarkable number of disallowed deductions from not following those instructions. To be fair, sometimes there are other shenanigans going on and the instruction failures are the easiest way for the IRS to attack. Nonetheless, there is nothing to say that the IRS will not use the precedents set in those cases on your client even though they are not trying to get away with anything.
To get a simplified list of what to know and implement, continue reading.