How to manage the difference of purchase item unit and sale item unit

2

you have two options

  1. create inventory with unit as litre and enter the purchase in litres itself even if its actually bbl.
    for example if 1 bbl is purchased you can manually enter the purchase as 158.987294928 litres.

  2. create two inventory items each for bbl and litre. use a production order to convert the required volume. for example, if you want to convert 1 bbl to litres, the finished good will be litre with quantity 158.987294928 and bill of material will be bbl with quantity 1.

read the guides section regarding production orders.