Getting error when trying to use OPENQUERY Msg 7321, Level 16, State 2, Line 1
I would suggest you change your original SQL Statement as follows: 

SELECT * 
FROM OPENQUERY([XXX], 'SELECT * FROM database.table where left(MBCUNO, 1) = ''a'' and (MBCONO=650) ' )


This has the two variable swopped around, and the like % exchanged for a left(mbcuno,1).  

Ensure that all your table &amp; column references are correct