Kiran,
For a PO to be created you need to have the following data
Document type,Comp code, Purch Org, Purch Grp, Material code, Qty, UOM, Net price, Additional charges if any, delivery date.
Please create a Custom Table with the above fields in your SAP system. you need to obtain the above data from your third party system through your middle ware system Biz Talk, and store the data in respective fields Please include date and time in the Custom table as fields.
Create a separate program in SAP system with Date and Time as selection fields.
The program should call PO creation BAPI and create a PO in SAP system.
You need to discuss and decide on the timings of the Interface and run the program as a Background Job to create PO in SAP system.
Hope this helps you.
Raghavan