I take your point about the meaninglessness of sizing up dimensional quantities without a referent. But sometimes the referent is inherently specified in different units. If you want to travel, with constant speed, no more than 10 miles - less is OK - then the time of your travel must be small - how small? - well, its product with your speed shouldn't exceed 10 miles. You could say, just divide 10 miles by the speed and use that as the upper bound, but that only works if the speed is fixed. If you're choosing between traveling on foot, on a bicycle, and in a car, you really are choosing on two different axes that are jointly constrained. So it is in my post: the second derivative times the donation is constrained, and the units work out. You can say "this works when the donation is small enough and the 2nd derivative is small enough" without comparing them to something in their own units, because the meaning of "small enough" is in that dimensional equation.

Besides, consider the following: why is it X that you're comparing Δx to? Sure, it's in the same units, but how is it relevant? In your analogy, GNI per capita is relevant to $1 because it represents the mean income I could expect to generate over the year. But note that you're not comparing $1 to the total GNI of the country, even though it's in the same unit, dollars, because the total population size, which drives that number, is not very relevant to the effect of $1 on one single person. With charities, how is the current endowment relevant to the contribution I hope to make with my own donation? It is not, after all, as if my goal was to maximize my donation's utility relative to other donors' in the same charity - because we stipulated that I'm only caring about the total absolute good I contribute...

Thanks for the suggestion about my wording - I'll try to make that example a bit clearer along the lines you propose.