Extra cheddar please

This person wanted to work for a non-profit for free, but the organization was so disorganized that they ended up losing millions. They really lost a good employee, too — this OP was so dedicated to the cause that they actually planned to work for them for three months entirely for free. First, the company insisted the OP shouldn't do that, telling them to send an invoice ASAP. However, they company then decided that the OP was over-billing them, and accused the OP of some pretty terrible stuff…