How concatenate variable inside fieldcat

Hello everybody.

I want to concatenate a variable inside of a fieldcat, how can do in this case?

fieldcat:

           'AAAA' 'AAAA1' 'Amount $' ' ' 'X' '14',

             'AAAA' 'BBBB1' 'Amount $' ' ' 'X' '14',

             ....

My variable should replace the dollar symbol ("$"). All this is printed in ALV report.

Thank in advance.