How you treat inventory changes really depends on how you want to show this on your reports.
A couple of ways to handle donations.
1. Sell the items to a "Donation" customer, at $0 (or some discounted price). You can use discount rules if you think this will help your clerks make the right discounts.
2. Add the items to a Purchase Order (Donation, Chuck Over the Bank, Marketing, etc.) with a negative quantity and then receive those items. This will remove them from your inventory, keep a record of what was given to Donations, and will not negatively affect your sales margins.
Either way works.