Lichatchila and Bidieved in Biblical Laws? Teruma

In Bamidbar 18:25f we read the laws of terumat maaser. That is, the Levites receive tithes, called maaser. From those gifts, they must in turn give a tithe of their received tithes to the Kohanim, just as the Israelites must give a tithe (maaser) of their earned wealth gained by farming and must give a teruma to kohanim. (see here for a good summary - scroll down to "Synopsis of Torah Laws relating to the Land.") The pasukim:

כה וַיְדַבֵּר ה, אֶל-מֹשֶׁה לֵּאמֹר.	25 And the LORD spoke unto Moses, saying:
כו וְאֶל-הַלְוִיִּם תְּדַבֵּר, וְאָמַרְתָּ אֲלֵהֶם, כִּי-תִקְחוּ מֵאֵת בְּנֵי-יִשְׂרָאֵל אֶת-הַמַּעֲשֵׂר אֲשֶׁר נָתַתִּי לָכֶם מֵאִתָּם, בְּנַחֲלַתְכֶם--וַהֲרֵמֹתֶם מִמֶּנּוּ תְּרוּמַת ה, מַעֲשֵׂר מִן-הַמַּעֲשֵׂר.	26 'Moreover thou shalt speak unto the Levites, and say unto them: When ye take of the children of Israel the tithe which I have given you from them for your inheritance, then ye shall set apart of it a gift for the LORD, even a tithe of the tithe.
כז וְנֶחְשַׁב לָכֶם, תְּרוּמַתְכֶם--כַּדָּגָן, מִן-הַגֹּרֶן, וְכַמְלֵאָה, מִן-הַיָּקֶב.	27 And the gift which ye set apart shall be reckoned unto you, as though it were the corn of the threshing-floor, and as the fulness of the wine-press.
כח כֵּן תָּרִימוּ גַם-אַתֶּם, תְּרוּמַת ה, מִכֹּל מַעְשְׂרֹתֵיכֶם, אֲשֶׁר תִּקְחוּ מֵאֵת בְּנֵי יִשְׂרָאֵל; וּנְתַתֶּם מִמֶּנּוּ אֶת-תְּרוּמַת יְהוָה, לְאַהֲרֹן הַכֹּהֵן.	28 Thus ye also shall set apart a gift unto the LORD of all your tithes, which ye receive of the children of Israel; and thereof ye shall give the gift which is set apart unto the LORD to Aaron the priest.

The famous phrase cited for this terumat maaser is כֵּן תָּרִימוּ גַם-אַתֶּם, "Thus ye also shall set apart a gift". The word אַתֶּם is used to restrict who can do it.

דף כט, א פרק ה הלכה ד גמרא

במדבר יח) אתם פרט לשותפין.
אתם פרט לאפיטרופוס.
אתם פרט לתורם שאינו שלו.
אתם פרט לשותפין.
והתנינן השותפין שתרמו.
אלא כאן בתרומה גדולה.
כאן בתרומת מעשר.
כלום למדו תרומה גדולה לא מתרומת מעשר?!
אלא כאן להלכה.
כאן למעשה.

אַתֶּם - "you (pl.)" - excluding partners
אַתֶּם - "you (pl.)" - excluding a legally appointed guardian
אַתֶּם - "you (pl.)" - excluding one who takes off teruma from that which is not his.
אַתֶּם - "you (pl.)" - excluding partners:
But we learned in a mishna: The partners who took of teruma...!
Rather here for teruma gedola (normal teruma) {partners can separate}
Here for terumat maaser {partners cannot, for the word אַתֶּם is written by terumat maaser.}
But do we not learn {the laws of} teruma gedola from terumat maaser?!
Rather, here for halacha {understood as the lichatchila, partners should not}
Here limaaseh {actual action - that is, after the fact, bidieved.}

The Korban HaEda explains that there is a gezeira, a rabbinic decree. In general, partners are not makpid, insistent, that one not take off for the other. If they were makpid it would be invalid. They made a gezeira on the general case because of the case when they are makpid. But, bidieved, after the fact, they would not invalidate it.

It is strange to talk of gezeira when after all the law is initially derived from a Biblical word - אַתֶּם! Perhaps that is the source for partners who are makpid. Still, the injection of rabbinic decree seems unwarranted here. Also, perhaps it is only teruma dirabbanan, but then the Biblical derivation again presents a problem.

On the other hand, it is generally accepted that there can be no lichatchila and bidieved in Biblical law. There are exceptions in the law of chalitza, and I've seen some in the laws of sota. This would present another counterexample, if it is indeed meant to be Biblical. One could take the possibly facile out and say that the derivation from אַתֶּם is an asmachta, a hint to a rabbinic law, but the law is not in fact derived from there. But I think this gemara is important as a possible counterexample to the assumption that there is no lichatchila and bidieved in Biblical law.