Commentary for Bava Metzia 4:1

הזהב קונה את הכסף – Each thing that is considered to be a coin and currently passing to be given in expenditure (of money), it has the law of coinage. But where it is not considered to be a coin and currently passing, it has the law of merchandise. And his taking possession by drawing towards one’s self the object to be acquired is the establishment of the matter. Therefore, from when one draws [towards oneself] the gold denar, the second has acquired the silver denar in whatever place they are, and neither one of them can retract in this, for the golden denars are [considered] produce in regard to the silver denars.

Gold acquires silver, but silver does not acquire gold.
Copper acquires silver, but silver does not acquire copper.
Bad coins acquire good coins but good coins do not acquire bad coins.
An unminted coin acquires a minted coin, but a minted coin does not acquire an unminted coin.
Movable property acquires coined money, but coined money does not acquire movable property.
This is the general rule: movable property acquires other movable property.
The first two mishnayoth of chapter four deal with what constitutes the finalizing of a transaction of movable property (movable property includes things and animals and does not include land). The importance of this halacha is that when the transaction is final neither side may retract the sale. For instance if an animal is sold and then dies before the seller can bring it to the buyer it is important to know if the sale was final. If the sale was final then the buyer’s animal died. If it was not final than the seller’s animal died. The general rule that is important to note in the outset is that the transfer of money does not cause the acquisition to be final. In other words if Reuven gave Shimon 100 zuz for his cow the cow does not belong to Reuven until he takes possession of it (this can be done in various ways).
As explained in the introduction, money does not acquire movable property, but the transfer of movable property does obligate the buyer to give the money. Our mishnah defines what money is in relation to movable property. To understand this mishnah one must keep in mind that in those times coins were based on their weight in silver and the authority of the government who had minted the coin. Most coins were made of silver but there were gold and copper coins as well. Silver coins were more easily accepted in the marketplace than gold or copper coins. Section one teaches that gold is “movable property” in relation to silver which is “money”. Therefore if the owner of the silver takes the gold from its owner he is obligated to give him the silver, even if he were to change his mind. If, however, the owner of the silver gave the silver to the owner of the gold, the sale is not final and the owner of the gold may still retract the exchange.
The remainder of the mishnah similarly defines property vis a vis money. Copper is “property” compared to silver. Bad, worn out and unminted coins do not fall into the category of “money” but are rather closer on the spectrum to being “movable property”. Finally, if two pieces of movable property are being exchanged, the acceptance of one creates an obligation for the other to be given as well.
Examples relevant to this mishnah will be given in the next mishnah.
 What might be a possible connection between those of the generations of the Flood and the Dispersion and those who don’t keep their word?

והכסף אינו קונה את הזהב – since the silver denars are currently passing in expenditure [of money], they have the law of money with regard to golden denars. And if one [of them] pulled the silver denars, the other did not acquire the golden denars, until he pulls [the golden denars], for the money does not purchase, and that is the reason also why cooper acquires the silver, for pennies of copper whose passing is not so important [in the expenditure of money], they are produce in regard to the silver denars, and the silver does not purchase the copper.

מעות הרעות – which became invalidated

אסימון – that was made in the manner of current coinage but it still had not stamped upon it a feature.

כל המיטלטלין קונין זה את זה – if he exchanged these for those, since the one that pulled/drew towards one’s self, the other fellow acquired [the other object]. And this word כל/”all” – includes even a purse filled with money [in exchange] for a purse filled with money.