A pele-dike Ramban: He says [Mitzva 17] based on the pasuk "ולא ירע לבבך - your heart shouldn't be sad when you give tzedaka", that there is a separate mitzva, one of the 613, that one must believe that he is not losing by giving tzedaka because it will only ADD to his money!! In that way he will give with an עין טובה - a generous "eye". His language is:
"אבל יהיה זה בעינינו שכר ותעולת ותוספת בממוננו"
How do we know that one doesn't lose by giving tzedaka? The pasuk states "כי בגלל הדבר הזה יברכך ה' אלקיך".
The Rambam and Chinuch argue - not with the principle of the Ramban that one must give with simcha - that is a pasuk ולא ירע לבבך - but that it is not a separate mitzva but part and parcel of the mitzva of tzedaka. So if someone gives 10 million dollars for a building but inside it HURTS - he is not properly fulfilling mitzvas tzedaka.
Wonder of wonders.
He is section from the amazing sefer דרישת פיקודיך by Rav Kunstler Shlita.