It is interesting to contrast the Chinukh’s description of terumah, the first gift taken from one’s produce, given to a kohein, with his description of ma’aser rishon, the tenth given to the levi’im.

Terumah, mitzvah #507:

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משרשי המצוה, לפי שהדגן והתירוש והיצהר הן עיקר מחיתן של בריות, והעולם כולו להקדוש ברוך הוא הוא, על כן ראוי לאדם לזכור את בוראו על הברכה אשר ברכו ושיפריש קצת ממנו לשמו ברוך הוא, ויתננו למשרתיו שהם הכהנים העסוקים תמיד במלאכת שמים טרם יגע בו יד אדם ויהנה ממנו כלל. ומן היסוד הזה אמרו זכרונם לברכה שאפילו חטה אחת פוטרת את הכרי, כי זכירת האדנות על הדבר אין הפרש בין רב למעט. אמנם רבותינו זכרונם לברכה הוסיפו בדבר לתת בו שיעור ראוי כדי שיתעורר לב האדם בענין יותר…

From the roots of the mitzah: Because grain, vine and olive crops are the essence of what keeps people alive. The whole world is the Holy One’s (blessed be He). Therefore it is appropriate for a person to remember his Creator for the blessing which he blessed him, and separate a little of it for His sake (blessed be He), and to give it to his servants — which are the kohanim who are constantly busy with the work of heaven — before anyone’s hand touches it and gets any benefit from it. From this foundation, [our Sages] of blessed memory said that even one wheat stalk can permit an entire stack. Remembering the Mastery [of G-d] with something makes no difference whether it is a lot or a little. However, our Rabbis of blessed memory added in one thing to give it a [minimum] appropriate amount in order to awaken people’s hearts on this subject…

Terumah is  described as something between man and his Creator. In order to acknowledge that the crops come from Hashem and really belong to Him, we are obligated to give some to His representative, the kohein, before enjoying any ourselves. Torahitically, there is no amount that needs to be given because terumah is about the act of giving which is required .

Ma’aser rishon, mitzvah #395:

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משרשי המצוה, לפי ששבט הלוי בחר השם בתוך אחיו לעבודתו תמיד במקדשו, על כן היה מחסדו עליהם לתת להם מחיתם דרך כבוד, כי כן יאות למשרתי המלך שתהיה ארוחתם מזומנת להם על ידי אחרים שיכינוה להם ולא יצטרכו הם ליגע בדבר זולתי בעבודת המלך היקרה. ואף על פי שהם היו שנים עשר שבטים, ולפי חלוקה שוה היה ראוי שיטלו חלק אחד משנים עשר, גם זה היתרון להם לכבודם, כי מהיותם מבית המלך ראוי שתהיה חלקם יתרה על כולם. ויתרון גדול הוא שיבא להם חלק העשירי נקי מכל הוצאת הקרקע. והמחיה משרתי האל בממונו ברכת השם יתברך תנוח עליו בכל אשר יש לו, וזהו אומרם זכרונם לברכה [אבות פ”ג מי”ג], מעשרות סייג לעושר….

From the roots of the mitzvah: Because the tribe of Levi was chosen by Hashem from among his brothers to serve Him constantly in His sanctuary. Therefore, He bestowed a kindness on them to give them their livelihood in a respectful manner. For this is what’s appropriate for the servants of the king, that theyir meals would be prepared for them by others and they would not have to think about anything except the precious service of the King. Even though there were twelve tribes, and by equal distribution they should have gotten one twelfth [rather than a full tenth], this extra too is for their honor. Because their being of the King’s household makes it appropriate for their portion to be greater than everyone else’s. An even greater extra is that this one tenth portion is given to them without any fieldwork. One who provides for the servants of G-d with his money, a blessing of Hashem yisbarakh rests on him in whatever he did. This is what [our Sages] of blessed memory said, “tithing [maaseros] is a fence around wealth [osher]…”

Ma’aser is about supporting leviim, so that they have no tasks other than serving their King. It is therefore about the food given, and has a set amount — one tenth — as befitting the dignity of their role. Unlike terumah and its acknowledgement of the Source of all crops, ma’aser is an interpersonal mitzvah.

We can also use this distinction to explain why leviim can forgive the debt of their maaser, but a kohein can not do the same for terumah. Terumah isn’t the kohein‘s, he is eating “from the King’s table”. The point is the farmer or buyer’s need to give, not on the recipient.

Similarly, we can also suggest that this is why terumah is given before ma’aser. It serves a similar role to berakhos. Until we acknowledge Hashem’s role in creating the crop, it is not ours to give to the levi’im.

One is about acknowledging G-d, and the primary gift is the giving itself. The other is about supporting those who serve Him, and therefore about the food they require. Two similar seeming mitzvos that are at their root, very different things.