I think you should give them permission in writing to reproduce the painting free of any fees for the particular print edition they're doing. Just state clearly that you're waiving any fees for reproduction rights in this case and describe specifically what the print edition is (size, number/unlimited,etc.). Don't worry about it sounding petty because you never know what will happen in future. If they ask point out to them that it could work to both of your advantages because it leaves open an option for future projects and defining the edition is important for your integrity as an artist and your records. Do state that you retain the copyright - you may want to do different reproductions later or even do a different type of product (mugs? t-shirts?
) later (for your own profit or another project to help the building, you never know).
You retain the copyright even if you give them the painting to sell at auction. For the sale remember to get a good photograph/slide of the artwork for future use before it's sold. (again, also useful in case you want to do editions later) You should be able to make a tax deduction based on the final auction price too, depending on US rules and if it's a registered charity or public institution. Check it out.
Hope it makes them bundles of money and you save the building!!!