Your curl command looks right to me:
curl -k -u user: -X POST -H "Content-Type: application/json" -d @"unmanage.txt" "https://x:17778/Solarwinds/InformationService/v3/json/Invoke/Orion.SEUM.Transactions/Unmanage"
["T:2","04-26-2017 12:00:00 PM","04-27-2017 02:00:00 PM","false"]
And it did not return an error. Are you saying that the transaction was not in fact unmanaged after doing this?