Sorting a DMEE tree by a CURR data type field

I need to sort the items in the SEPA XML file by internal currency amount (field FPAYH-RBETR, I think) descending.

At this moment I'm using a custom DMEE tree with the following Sort-fields:

Key field  No sorting  Sequence  Structure  Sort field  Level

   X                X           1                FPAYH   ZBUKR     1

   X                              2               FPAYH   AUSFD      2

   X                              3               FPAYH   HKTID       2

   X               X             4               FPAYH   DOC1R     3

I tryed to add FPAYH-RBETR field, but I got na error message, bacause CURR data type is not allowed.

Can someone give a clue on how to solve this problema?

Thank you

Best regards