Linking transaction and Master data

Hi,

I have my transaction data coming from three sources( three different transaction data from three different sources) The key for the transaction data is A,B,C. One of my dimensions has A,B,C,D as keys.This dimension is associated with all the transaction data. How do i link it to the transaction data so that i dont get multiple master data entries for a given transaction record.

It is not possible to provide the KEY D in transaction data. Is there a way out?

Thanks