Development Completed :- 
Added New attributes in Product Variant and Patches (firstReceiptDate and lastReceiptDate ) in SP and API . Tested in CT UAT.
Added a new field in existing NAV Send Order JSON payload named as 'multiBuyPromo' and capture the value from below Product's attribute from the Fluent JSON response file ("fluentFulfilments" --> MultiBuyPromo: True/False).Development Completed .