I would like to suggest that you add a Tithing Option to manage the amount of money one wants to share.
It would have a percentage option, so that users can set how much percent of their earnings they want to give to charity, and maybe also an account selection so one can choose from which account the money will be coming.
A tithe invoice section so users can set their charities etc. and manage the amounts they gave.
Then a little report page where one can see how much he has already given, and how much he still needs to give.
Thanks for considering the above.