Significance of Management Business Unit on Item Organization

Hi - We need to maintain sales account at item level and based on that we need to derive Revenue account. Now in our case sales account at item level is same across all inv orgs and hence we need to maintain at item master org so that it will cascade to all inv orgs pointing to the master org. Now we have defined master org in Manage Item Organization task without associating any Management BU, hence we are not able to store sales account at item level (the field is greyed out). As per oracle if item org is