This command allows us to report(!) the reward donations to facebook, however, the donation will be made through a channel out side of FB (i.e., our page or similar). However, it would be credible for the user/participant that the donation comes from us (anonymously) and is received by the charity she chose.

This is where we can implement the message treatment

Is it necessary that the FB fundraiser "mirrors" a fundraiser on our page? Or can it be Independent? If not, can we reduce the content of fundraisers on our page to proxies.

This indicates that each FB fundraiser is linked to (or mirrors) a (proxy) fundraiser on our page. It also indicates that donations to the proxy are possible and that the amount might be visible on facebook.