terumah lishma - explanation of the shev shamytsa

Last year Chaim Markowitz beat me to posting a beautiful Shev Shmaytza on the opening of this week’s parsha which we had discussed together 16 (well, now 17) years ago. Rashi comments on "v’yikchu li terumah" that terumah for the mishkan had to be given lishma. Tzedaka is acceptable even if done for ulterior motives – why here does the Torah demand donations be given only with pure motives? See his post for the Shamytza’s answer.