Hi Angi,
while we have never offered anything back to the original donor's after
deaccessioning it (in the case where the donor got a tax receipt, I'm
not sure you can legally give it back to them).
In terms of time limits, we usually post the list of deaccessioned
artifacts to several internet newgroups and give the museum community 30
days to respond with expressions of interest. You might have to allow
more time because of the snailmail. What did your solicitor advise?
Richard Gerrard