Formula will omit sending the field to the destination system
##BlankValue(DBLookup('DESTDS', 'User', 'Alias', "Salesperson", 'Id'), SkipField())
This will send the Id of the User with the alias as in the Salesperson field only when this user exists in the destination, otherwise, it will skip the field and not send it to the destination at all (leaving the value that is already set or letting the destination generate a default value if it is a new record). This could be used with MS Dynamics NAV Customer as the source and Salesforce.com Account as destination.
##IF(EQUALS("Type",2), DBLOOKUP('DESTDS','product','productnumber',"No.",'productid'), SkipField())
This will send the productid corresponding to item "No." to the destination for lines with Type = 2 and it will skip the field for all other lines. This could be used with MS Dynamics NAV invoice lines as source and MS Dynamics CRM invoicedetail as destination.