How to Manage FIFO rule for Costing in a Point of Sale ?

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VB

SQL-Server-2008R2

Hello to All !

Dear fellows i am working on a Point of Sale project for a Trading Company in VB.NET 2010 and SQL Server 2008.

My Client asked me to manage FIFO rule for Costing of their items.

The requirement is that when an item is sold in a large quantity let say 10000 units then system should charge the cost according FIFO rule.

Now i explain in detail.

In above example when 10000 units of a same item are sold and system has 12000 units available in stock but these 12000 units were purchased in four different creditors at different purchase rates i.e.
in 1st purchase 3000 units purchased @ 110/unit
in 2nd purchase 3000 units purchased @ 115/unit
in 3rd purchase 3000 units purchased @ 120/unit
in 4th purchase 3000 units purchased @ 125/unit

now when 10000 units are going to be sold i want that system should charge the cost of these 10000 units of the item as following

3000 units @ 110/unit (from 1st purchase) +
3000 units @ 115/unit (from 2nd purchase) +
3000 units @ 120/unit (from 3rd purchase) +
1000 units @ 125/unit (from 2nd purchase)

how can i manage it ?

if anyone has done the same practice he must have the solution.

thanks in advance !