Different Units for Purchase and Sale

Different Units for Purchase and Sale


I have strange question.

I want to use Books for my Purchases and Sales with Inventory (not the Advanced Inventory). I am into Aqua Culture.

I buy baby fishes - let say 100 nos. These are grown in our ponds where they grow and become adult fishes.

Now I sell them based on the Weight and NOT on the quantity.

Please suggest me on how to set up my Books so that I can use 
- Qty of Child Fish as Item Unit in Purchases and 
- Fish Weight as Item Unit in Sales.

Also when a sale happens the inventory count should reduce to the total number of fishes sold.

I hope someone will have an answer for this...