In a pinch, maybe it was the only example to bracket some particular feature?
My method, if I had to illustrate that sale for whatever reason (rural market with minimal transfer activity???), would be to leave it without a PFA adjustment and merely state that it represents the lower range of value, wasn't arm's length because (place whatever the reason was here) and that it really doesn't meet Fannie's definition for a proper comparable. It's additional information, and it's the best I had. I didn't weight it in the final reconciliation, blah blah blah. Paraphrased for brevity here, of course, but you get the idea.
In the sales grid, were the adjusted values in a tight range with your other comparable falling $25,000 lower, you really have a paired sales analysis there in the grid. BUT- it only applies to that particular sale. Non-arm's length transactions take place all the time, but the difference above and below typical market value are completely unrelated from one transaction to one another. It's not like there is a typical 'she's my sister' discount in your market of $25,000. It all depends on the circumstances surrounding the deal and the level of generousity, desperation, or greed as it may be of the selling party.
I suppose you could loosely call it a concession, but standard appraisal practice, ie. what your peers would normally do, is avoid the comparable if at all possible.