Franking personal letters was badly done. I did not look at selling the Mansfield living from a religious point of view. Giving or bequeathing a living is certainly on the side of kindness and generosity whereas selling a living does not seem as in keeping with religious beliefs. Mr. Norris received the Mansfield living as a gift, ‘Sir Thomas being happily able to give his friend an income in the living of Mansfield’ (1) and had not intended to sell it after his friend’s death:

The living was hereafter for Edmund; and, had his uncle died a few years sooner, it would have been duly given to some friend to hold till he were old enough for orders. But Tom’s extravagance had, previous to that event, been so great as to render a different disposal of the next presentation necessary, and the younger brother must help to pay for the pleasures of the elder. (3)

I do not think Sir Thomas was selling the Mansfield living to serve greed but rather out of necessity. I looked into the L&T archives and found two different takes on the ethics of selling livings (advowsons). I have copied only a snippet from each post but they each are very interesting and informing. I have listed the title and address of each if anyone wishes to read the posts in their entirety. I highly recommend it:

Of course you may ask this question Written by JulieW (April 22, 2005)
http://www.pemberley.com/bin/archives/regarc.pl?read=40595

Thought the practice of buying and selling advowsons might seem at odds with our 21st century sensibilities, it was perfectly acceptable to the majority of the clergy and laity of the time.

Right of advowson Written by Caroline (August 20, 2002 )
http://www.pemberley.com/bin/archives/regarc1.pl?read=26490

They literally bought the approval of the owner of the advowson... Although it wasn't strictly illegal, it was considered rather disgraceful, and it's obvious in MP that Sir Thomas would not have done it if he hadn't needed the money in a hurry.