Rule 3: Where there is a sale of specific goods, but the seller has agreed to weigh, measure, test, or do some other act to the goods for the purpose of determining their price, property passes when the thing is done and the buyer has notice that is has been done.
Example: B agrees to buy 5 boxes of mushrooms from S. The price of the mushrooms is $100 per kg. S has to weigh the 5 boxes of mushrooms to determine the price, and inform B of the price, before property in the mushrooms can pass.