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It is extremely common when re-selling goods, that a purchase of inventory is made in one unit of quantity, but when sold a smaller unit of quantity is used.

For example, you might purchase 1 whole box of something containing say 10 units, but sell the units in the contained units. For example, I buy plastic filament by the roll, containing a qty of N Kg, but when its placed on a customer invoice or quote, the qty is in grams. I buy by the Kg, and Sell by the gram. But others might buy ONE box of screws containing 100 of them, The purchase price is $$ PER BOX. I currently enter in the sale quantity, and divide the purchase price by the number of units, but, due to mathematical precision of only 3 decimal places, the price calculates incorrectly, and I have to adjust it later.
So, the suggestion is to make Billing or "Buy Now" transactions with two quantity columns, one being the quantity purchased (linked directly / to the exactly purchase price), and the second column being the qty available for sale, and this is what is tracked in Inventory. The unit of sale and its devolved price is used for Quote, Project and Invoice transactions.