Store.CurrentMember.Properties("Store Manager")

SELECT 
{ [Time].[1998] } ON COLUMNS , 
{ Descendants([Store].[All Stores].[USA],3) } ON ROWS  
FROM [Sales_MDX2] 
WHERE ( [Measures].[Store Mgr] )

Filter(Store.[Store Name].Members, Val(Store.CurrentMember.Properties("Store Sqft")) < 21000)