זכר למחצית השקל
The רמ"א in תרצ"ד brings the מרדכי who says we should give 3 half shekels on ערב פורים. The reason to give 3, explains the רמ"א, is that it says the word "תרומה" 3 times in פרשת כי תשא. רש"י there explains that these 3 "תרומה"s correspond to 3 different collections: 1. The half shekel for קרבנות ציבור, 2. Half shekel for the אדנים, and 3. The מגבית of "כל נדיב לב" used for the משכן itself.
The גר"א in מעשה רב says to give 1 half shekel. The כף החיים agrees, and explains that the מרדכי’s idea of giving 3 is shver. Why? Because the רמ"א himself says that the purpose of this נתינה is a זכרון to what was done in the past. פשטות, the purpose of making a זכר is to express a yearning. However this concept does not apply to all the 3 half shekels. Only the half shekel for the קרבנות was a מצוה לדורות which is שייך to yearn for. The other 2 תרומות, i.e. אדנים and “כל נדיב לב”, were one time collections for the משכן that will not come back again with משיח. So why should we make a "זכר" for something that won’t come back?
The גמרא says that the חטא that caused the גזירה of המן was the Jews’ participation in אחשורוש’s סעודה. The מפרשים explain that since the point of אחשורוש’s סעודה was to celebrate his belief that the בית המקדש would never be rebuilt. By participating in this ,סעודהthe jews were showing lack of חשיבות and even ,ביזוי for theבית המקדש . The jews were showing that they didn’t mind not getting the בית המקדש back. This ,חטא says the ,גמראcaused המן’s גזירה.
The ענין of "זכר" has another הבנה aside from yearning. "זכר" is also a means of showing כבוד and חביבות to something, even without asking for that thing to come back. Therefore, by doing a זכר to the מחצית השקל of the אדנים and the "כל נדיב לב", we are showing כבוד to the idea of theמשכן and"ועשו לי מקדש ושכנתי בתוכם". The point is not to ask for the משכן back, but rather to be מתקן the חטא of ביזוי המקדש that occurred during סעודת אחשורוש, by seizing an opportunity to show respect even to the משכן, which isn’t coming back. This is a further תיקון for the חטא participating in אחשורוש’s סעודה.
Making a זכרון to the תרומת האדנים and "כל נדיב לב" may seem like going to an extreme. Why isn’t our desire for בנין בית שלישי, as demonstrated by our זכרון to the מחצית השקל that was for the קרבנות ציבור- enough of a תיקון?
The רמב"ם in הל' תשובה ב:ד' says "מדרכי התשובה... ומתרחק הרבה מן העבירה". The מפרשים on the רמב"ם explain that the רמב"ם is saying this לשיטתו in הל' דעות where the רמב"ם states that in order to correct a מידה, one should go to the opposite extreme. The same מהלך applies to other עבירות as well.
Based on the רמב"ם’s יסוד in how to do תשובה, we can understand why it is necessary to make a "זכר" to the collections for the משכן which will not return. As a means of being fully מתקן this חטא, we need to go to the other extreme of כבוד המקדש. We do so, according to the ,מרדכי by giving the additional 2 half shekels זכר למשכן.