{Rambam laws of buying chapter 5 halacah 4 }

 If a person buys some goods, and then owes money. Then the buyer wants to sell stuff of his own {Rambam, Laws of Buying, chapter 5, halacah 4.} and then seller tells him, "Sell it to me, and your debt will be paid." At that point, the original seller does not have to do any act of acquisition. The Raavad as you can imagine disagrees with this (twice). The Gemara in Kidushin  says if this were so, then it would have to work for kidushin (marriage) also. And we know it does not. The Netivot has an answer for the Rambam and Rav Shach also. But  for now I just wanted to state what the problem is just for the sake of information. The problem is that in marrying a woman, one can not do so by a loan. This is  a famous halacha. That is to say: you loaned her money. Then you want to marry her. So instead of giving her a ring you say, "The debt you owes to me is forgiven." That does not work, and the reason is as the Gemara says הלוואה להוצאה נתנה  a loan is given to be spent. [That is the money of the loan already belongs to her.]

So what is the difference between this case and the above case of the buyer and seller? The buyer owes money. The seller then forgives the debt and the property of the buyer goes automatically to the seller.

I confess I did not understand Rav Shach's answer to this problem but I did see how he blew all the other proposed solutions out of the water. His seems to revolve on the money owed by the woman is forgiven as opposed to the money owed by the buyer which creates an act of purchase. I can see that the origins of the money owed in the two cases are different.One is in fact a loan. The other is money owed because of goods bought. Still I have  a hard time understanding why that would make any difference