False Information from customer service

My wife and I were promised 600 euro each if we would volunteer to take another flight because our flight was overbooked. The agent in Frankfurt lied to us because we could not redeem the voucher when we arrived in Boston. We sent the vouchers to NY and no response or a number to contact. I'll never use this airline again. What a bunch of liars and crooks- trying to collect on my voucher cost me $200 in wages.