Pricing of item in dual UOM: How to price an item in both primary and secondary UOM when UOM convers

A business produces and sells lamb carcasses.

Customers are priced in Pounds or Each. Price list is as follows:

5000 per Each (carcass) 

300 per Pound                  

There is no fixed UOM conversion rate between Each and Pound. On average one carcass weighs around 15 Pounds so that is the conversion rate defined in inventory

Scenario:

Lets say in one particular transaction a customer is sold one carcass that weighs 12 Pounds. Pricing is on per Piece basis ie 5000.

When we enter the order we change UOM to Each.

We allocate and ship 12 Pounds and 1 Each.

Problem:

System ships 12 Pounds and 0.8 Each.

0.8 Each is invoiced instead of 1 Each.