Giving Charity with an Ulterior Motive

The gemara says the if one gives צדקה so that his son will live or that he merits reward in the next world he is considered a צדיק גמור. The gemara in כתובות tells a story of the daughter of Nakdimon ben Gurion, who was one of the richest people in Jerusalem but after the destruction of Jerusalem this girl was seen picking barley seeds from animal excrement due to her hunger and poverty. Raban Yochanan ben Zakai asked her "what happened to all your fathers riches"? To which she answered "It was all lost because father did not give charity". The gemara asks that we know that her father did give charity? The gemara answers that although he gavecharity, he gave charity to acquire honor and not for its own sake. The Maharsha expounds that this gemara is teaching us that if one gives charity to get honored he gets punished for it. This seems like a direct contradiction to the earlier gemara that states that one MAY give charity with an ulterior motive. The answer is that yes, one may give charity with an ulterior motive, however, when one gives charity to get back honor then he is not giving charity. He is transacting. He is purchasing something
 When the wealthy man goes to the grocer and purchases potatoes he is not giving charity to the grocer with an ulterior motive to get potatoes, he is sply purchasing potatoes. When the wealthy man gives charity for honor he is going to the Rosh Yeshiva and effectively saying " I have a surplus of money but need honor, you gave a surplus of honor but need money, let transact. That is not charity with an ulterior motive and that was Nakdimon ben Gurion's issue. He didnt give charity but was a purchaser of "Kovod"