I am. Each time a different batch record is selected I have as default process that updates the attribute for LIRU_PS_SelctedDonationHdr.
I just tried this condition as well:
LoggedInRegularUser.PS_SelectedDonationHdr.BatchPosted='No' OR LoggedInRegularUser.PS_SelectedDonationHdr.BatchPosted IS UNDEFINED
And the Add button o the Donation Detail Header still displays.