The mind of C. S. Lewis never fails to awe me. In the 26th letter of The Screwtape Letters, he writes for Uncle Screwtape and says,

The grand problem is that of ‘Unselfishness’. Note, once again, the admirable work of our Philological Arm in substituting the negative unselfishness for the Enemy’s (that would be God for those who haven’t read the book yet) positive Charity. Thanks to this you can, from the very outset, teach a man to surrender benefits not that others may be happy in having them but that he may be unselfish in foregoing them. That is a great point gained.

The difference is that ‘Positive Charity’ will seek to be unselfish because it wants to see the other person happy, and ‘Negative Unselfishness’ seeks to see the other person get what they want because it is an opportunity for itself to display how ‘Unselfish’ it is. ‘Unselfishness’ is based in pride, and the other real love.