Daily Inventory Balance (using formulas)

[MrVillareal]

Hi there!

I need a help regarding inventory balances daily as of the day of transaction. Please help.
I have 2 worksheets, one is purchases and the other is sales. Both worksheets has the same row field (date,item,quantity). For instance,

For purchases sheet:

Date Item Quantity (Inventory Remaining)
1/1/2011 a 10 ?
1/2/2011 a 10 ?
1/3/2011 b 10 ?
1/4/2011 a 10 ?

For sales sheet:

Date Item Quantity (Inventory Remaining)
1/1/2011 a 1 ?
1/2/2011 a 1 ?
1/3/2011 a 1 ?
1/4/2011 b 1 ?
1/5/2011 a 1 ?

I want to have an additional row field for inventory balances both in the purchases and sales worksheets to determine my inventory balance as of the day of transaction whether its a sale or purchases transaction. Please consider also the date for duplicate transaction.

Thank you and God Bless!

MrVillareal