Parshat Pekudei: Why is silver for the Mishkan sourced differently than gold and copper?

Earlier, in Parshat Terumah G-d, via Moses, had called for the voluntary contribution of various precious and other materials needed for the design and construction of the משכן, the Tabernacle. Only those men who were twenty or older and had served in the military were even eligible to make these contributions which included gold, silver and copper.

In Parhat Pekudei the word תרומה is replaced by hתנופה, which are not entirely dissimilar,the former being a ‘lifting’ the latter more of a ‘swinging’ which involves lifting as well.

Yet the odd thing is that in Peukdei both gold and copper are referred to as תנופה (i.e. תרומה) whereas silver is not. Here instead of the silver being a part of that purely voluntary תרומה giving, we are told that the silver – used for the base sockets and tent hooks – came from the half shekel (מחצית השקל)that every male Israelite (i.e. age 20 or older and an army veteran) had to contribute as part of the census count.. Moreover this aggregate sum was precisely enough for the specified purpose of creating base sockets and hooks to hold the entire structure fast.

Now since the silver did not come from תרומה – and this is spelled out very clearly in our parsha here – what then happened to any silver that was indeed donated as part of the תרומה, and why was that silver not used for the construction project. For if there was surplus silver one would imagine that the raw silver given as תרומה would be smelted for use as sockets and hooks while the cash raised via מחצית השקל would be kept for the Mishkan treasury to be held for future purchases.

I would like to suggest – and this is merely a suggestion – that the half shekels were used in this way in order to give every proper Israelite ‘איש’ the feeling that he had a personal stake in the Mishkan’s ability to stand straight with its covering held fast in place. Moreover, by using only the מחצית השקל no single person could claim that he had a greater role in assuring the sturdiness of the Mishkan structure.

If anyone has another – of hopefully a better – explanation, please do let me know