There is no מנה here; there is no collateral here - מנה אין כאן משכון אין כאן
If a man intends to be מקדש a woman with a מנה and tells her that he will (eventually) give her the מנה, and now he is giving her a משכון as a guarantee of his intention she is not מקודשת since מא"כ מא"כ; meaning that since he did not give her the מנה with which he intends to be מקדש her, rather he merely gave her a guarantee, it is ineffective קידושין, since קידושין requires a transfer of כסף. However if he gave her the משכון as קידושין and stipulates that when he has the money she will return the משכון, then she is מקודשת, since he is giving her something of value at the time of the קידושין.
Concerning promising a gift, the same rule applies that a משכון cannot create an obligation where none existed before, and the intended beneficiary is required to return the משכון to the owner. However if the benefactor clearly stipulated that the beneficiary should not return the משכון until the benefactor gives the promised gift, then he may keep the משכון.
There are exceptions to this rule of מא"כ מא"כ, notably where the משכון is given to prevent a possible monetary loss or embarrassment to the recipient as in the cases of workers (leaving their jobs) and (recanting) שידוכים.
It is another’s משכון; and according to ר' יצחק, etc. - משכון דאחרים וכדרבי יצחק
The rule of בע"ח קונה משכון (regarding גניבה ואבידה) is by a משכון שלא בשעת הלואה, nevertheless even by a משכון בשעת הלואה the מלוה has a sufficient lien on it that he may use it for לקדש אשה (and other קנינים), and it exempts him from שמיטה, and makes him liable for בל יראה.
We derive that בע"ח קונה משכון from the fact that the תורה writes וברכך; if the בע"ח would not be קונה the משכון, the ברכה would be considered רבית דברים. Alternately, צדקה is only when it is from something that one owns.
Give it to him; she is not betrothed - תנם לפלוני אינה מקודשת
The details in one section of a ברייתא relative to each other should not be superfluous; however, relative to other sections of the ברייתא, the details are irrelevant as long as the sections as a whole are not superfluous. People may trust (certain) strangers more than their (prospective) in-laws.