The gemara says that if shimon stole a field from reuven and sold it to levi, if afterward shimon would purchase the field from reuven to maintain his sale to levi, levi would automatically own it without any kinyan. The gemara tries to understand what exactly would create the kinyan since the original contract is completely worthless. It is clear from the gemara that a kinyan made with a contract is effective by the seller handing it to the buyer. If at the time it was handed to the buyer it was a worthless document because the seller didn't own it at the time, it cannot go into effect afterwards just by being in the hand of the buyer.
Tosafos raises a question, why can't the money that was given by levi to shimon originally to acquire the field make the kinyan now that shimon purchased the field and now has rights to sell it. Tosafos says that this would be a problem of "mekadesh b'milveh". Just as one cannot marry a woman with a loan, one cannot purchase a piece of property using a loan. It seems to me that this answer only works according to Shmuel who holds that we view the money that levi paid to shimon as a loan, but according to Rav who views it as a pikadon, levi should be able to use it as a kinyan on the property. Does Tosafos answer only work in Shmuel?
Another point is that Tosafos explains that if a loan can create a kinyan, the money that levi paid shimon originally for the field would immediately be effective as money to acquire the field without having to speak anything out. Whereas, using the "hana'ah" of being mochel on a loan to create a kinyan, levi would have to speak out that he is being mochel the money to shimon and making a kinyan on the field with the hana'ah that shimon would receive. Therefore, a difference between acquiring with "a loan" and acquiring with "hana'ah of mechila on a loan" is that the latter must be spoken out explicitly.