You created an Extension Field in the Sales Order but the value is not copied when creating a Down Payment Invoice Request from the Sales Order.
SAP Business ByDesign
There is no extension scenario from Sales Order to Down Payment Invoice Request available.
The Extension Scenario Sales Order - General Information to Invoice Requests - General Information can be only used for Invoice Requests based on a Sales Order such as a Sales Order Invoice Request
This is the current system behavior.
Down Payment Request; Extension field , KBA , AP-CI-CIR , Customer Invoice Request , Problem