One class to build them all...

admin.py

class TransactionAdmin(admin.ModelAdmin):
    fields = ['date', 'bitcoin_address', 'payment_method', 'amount', 'email_address', 'completed']
    list_display = ['id', 'date', 'bitcoin_address', 'payment_method', 'amount', 'email_address', 'completed']
    readonly_fields = ['id', 'date']
    search_fields = ['^email_address', '^id', '^payment_method']
    list_filter = ['payment_method', 'completed']
    list_display_links = ['date']
    inlines = [TransactionDetailAdmin]
    actions = ['really_delete_selected']

    def get_actions(self, request):
        actions = super(TransactionAdmin, self).get_actions(request)
        del actions['delete_selected']
        return actions

    def really_delete_selected(self, request, queryset):
        for obj in queryset:
            free_address(obj.bitcoin_address)
            obj.delete()

        if queryset.count() == 1:
            message_bit = "1 transaction was"
        else:
            message_bit = "%s transactions were" % queryset.count()
        self.message_user(request, "%s successfully deleted." % message_bit)
    really_delete_selected.short_description = "Delete selected transactions"

    def save_model(self, request, obj, form, change):
        if obj.completed:
            free_address(obj.bitcoin_address)
        else:
            obj.bitcoin_address.in_use = True
            obj.bitcoin_address.save()
        obj.save()