As I understand, you have a date field that has 'sy-datum' type in web service method. When the method has been called from .NET side, date field is sent like '02/04/2014'. If you define date field type 'sy-datum or datum' (8 CHARS) your RFC will try to get the value like '02/04/20'. Therefore, your function will get a dump.
Please be sure that the value sent to your function is same format with yours.
In your case, value should be like '02042014' (8 CHARS - DDMMYYYY) or '20140402' (8 CHARS - YYYYMMDD).