In the Rambam laws of sale chapter 5 he brings the law of exchange. but in the case of acquisition by money in chapter 1 he does not bring the law of exchange. It seems clear from this that exchange of objects is not acquisition by money. Even though that which has monetary value is sometimes considered to be like money, still there are cases where it is not, for example the redemption of a Jewish slave has to be through money, not that which is worth money. Other examples are in laws of theft where there is an argument if one can pay by that which has monetary value. So when the Gemara says one cannot marry a wife by exchange that means exchange by vessels like a handkerchief, not exchange equal for equal. Here I am just mentioning in short what Rav Shach says in the beginning of laws of marriage.------------------------------------------------------------------------------------------------------------------- In the רמב''ם הלכות מכירה פרק ה' 5 he brings the law of exchange. But in the case of acquisition by money in chapter א' he does not bring the law of exchange. It seems clear from this that exchange of objects is not acquisition by money. Even though that which has monetary value is sometimes considered to be like money, still there are cases where it is not, for example the redemption of a Jewish slave has to be through money, not that which is worth money. Other examples are in laws of theft where there is an argument if one can pay by that which has monetary value. So when the גמרא says one cannot marry a wife by exchange that means exchange by vessels like a handkerchief, not exchange equal for equal. Here I am just mentioning in short what רב שך says in the beginning of laws of marriage.