If the EDT behind the field is correctly configured as a Number sequence reference in the number sequence module classes (SalesId is like that), DIXF would use number sequences correctly once you set the field as Auto-generated, if you send in a blank value for that field.
Refer to the official example from Microsoft:
blogs.msdn.microsoft.com/.../dixf-importing-data-how-to-use-auto-generated-to-import-partynumber