On Kiddushin 7a, Rashi writes (d.h. וכן לענין ממונא) about 3 principles where monetary transactions are supported by either the din of the guarantor (ערב) or the din of the eved ca'ani, or by both together.
If someone says "Give money to piloni, and my field will be sold to you" then the field is sold. Although the seller doesn't get any money, he is still obligated in the sale, just as in the case of the ערב, although the ערב doesn't benefit from the loan, he is obligated to repay (if the original borrower defaults). Ok, I get that.
If someone gives money to the seller of a field, and says the field belongs to piloni, then the sale is valid. In the case of a Caananite slave, if a third party gives money to the master for the release of the slave, the slave is free - the slave benefits even though he didn't use his own money. In our case here, the person who gets the field benefits even though he doesn't use his own money. I get this one too.
But I don't understand the third one. Rashi says:
תן מנה לפלוני ותקנה לו שדי בו קונה מדין שניהם
Why would the seller of the field have the money given to the buyer? Then the buyer gets the money AND the field. How is that a proper sale? And, why would the seller want the buyer to get the money?
Thanks for your insight. - Steve