I have one UN employee client for the last 10 years or so. UN should reimburse for full tax liability. If you’re getting the same summary letter, it should still say to report the income on line 7... the letter hasn’t ever been updated since I’ve been doing his taxes.... 

Verify  with your client the UN will reimburse the taxes... report the gross amount on sch c. Don’t take the 199a deduction and don’t take any expenses.