Hi Dennis
This is not possible as standard through the DTW templates or DI API. It must be totally unallocated or specifically allocated. The reason is quite simple. In SAP you can sort on most columns and then select the Add in Sequence function. This means that the invoices, etc. that are applied can be changed by sorting on date or amount for example. It would be difficult to replicate this in the DTW.
What you can do, is to add a query in Excel using ODBC that will get the open invoices. In your query, set the sort sequence and selection criteria. Once the open docs are in a spreadsheet, you could write a simple macro to apply an amount from say another workbook or worksheet to the open documents sheet in a row by row sequence till the amounts per customer are exhausted.
Then simply copy this data (or save as) to a csv file and use the DTW as normal. You could also write a macro to post direct from Excel using the DI API, eliminating the need for the CSV file and DTW.
Kind regards
Peter Juby