Some people have demonstrated how telling a very similar lie, but for any case other than a "good cause," is clearly unethical. I definitely agree with that

As much as I'm reluctant to say it, though... I think "The Ends Justify The Means" comes into play, though. If hypothetical jkafuman didn't mention that he would ALSO donate that same amount if you DIDN'T donate, I wouldn't blame him. (In fact I may thank him, possibly.)

(I'm not advocating purely utilitarian morality - I think the morality is ultimately a balance between "don't mislead people" and "save lives from malaria")