Hey there! 'Concern', when used as a verb, equals to 'have to do with or relate to' / 'be of interest or importance to' / 'engage the attention of someone (involve).' In this context, I should think that ''concern'' gets the meaning of ''a matter that relates to or affects one.'' So, you might get this idea: ''The question 'under discussion' is money.'' = In the expression 'in concern', ''concern'' is used as a noun (not as a verb) and the 'money' is the subject that is being discussed. Hope that helps!