How to skip mapping for a particular value in mapping

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Hi All,- 

I have a one requirement where i have to create requisition using SOAP web-service. The scenario is end user will send supplier item number  and i am directly mapping  to the supplier item number of request payload of create requisition.Now i have a condition  if supplier item number is 'Research' then i don' t need to do mapping to the supplier item number(I can't pass null) of request payload and i need to do mapping for other supplier item number.So is there any idea how to achieve this in OIC mapping