Complaint Review: lufthansa airlines

This is regarding the disputed booking number YX6QVP.
We changed the date due to a missing document. The airline manager at the gate did not explain the charges to be incurred. In fact we were told of a net 400$ refund after the price adjustments since next days fare was lower than the date we were originally traveling . We were surprised and dismayed at the turnaround by lufthansa as to not honoring the quote and refund. We were charged 3100$ additional without explanation at the time of rebooking . We learnt that we were put in different fare class within Business Class itself at the Lufthansa office in Frankfurt and were not explained this at the time of rebooking.