For example, I have 100 of the same item in stock (same sku).  

20 items from vendor A at $0.50/ea and sales tax was paid at purchase

80 items from vendor B at $0.55/ea and no tax was collected at purchase.

My customer wants to purchase 90 items from me.  Obviously this will require items from both vendors so how will QuickBooks know the tax will be different for each since some was paid already?