Propose that the main difference should be in money (8)
I believe the answer is:
nominate
'propose' is the definition.
(nominating is a kind of proposing)
'the main difference should be in money' is the wordplay.
'difference' is an anagram indicator.
'should be in' is an insertion indicator.
'money' becomes 'note' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'main' with letters rearranged gives 'mina'.
'mina' put within 'note' is 'NOMINATE'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for nominate that I've seen before include "Propose for office" , "Propose - name" , "Put forward name for a mention" , "Pick as a candidate" , "Put forward (as candidate for election)" .)