1. Your question is discussed by the Gemara in Nedarim (48b). Rav Nachman there says that even if one gave an object to his friend only in order that he should give it over to someone else, this is considered a valid transaction. Rav Nachman proves this from the Kinyan of the Sudar, where one gives the Sudar to the seller only in order that he should sell to him his car, and yet the transaction is still valid. Rav Ashi rejects Rav Nachman's proof and says, "Who says that if the seller would seize the Sudar [and keep it for himself], that his seizing would not be effective?" In other words, Rav Ashi understands that it is possible that the seller does not have to return the Sudar.

2. However, the Ran in Nedarim (49a, DH ul'Inyan) writes that the Halachah follows the view of Rav Nachman, that the seller cannot hold on to the Sudar. Even though Rav Ashi says that he might be able to keep it, this was merely a "Dechuya," a way of refuting Rav Nachman's argument, but in fact Rav Ashi himself also agrees that the seller may not keep the Sudar. This is also the way the Rema (CM 195:4) rules, that the seller may not keep the Sudar for himself.

3. However, if Shimon is Mochel, there seems to be no reason why his Mechilah is not effective. If Shimon wanted, he could have given the pen as a present to Reuven, so why should he not be able to say to Reuven that there is no need for him to return it when he gives it as a Kinyan Sudar? If, after Reuven has completed the sale of the item through the Kinyan Chalipin, Shimon now asks Reuven to lift up the pen again with the intention of acquiring the pen as a gift, this certainly would make it belong to Reuven.

Kol Tuv,

D. Bloom