Finally, I think we should not change purchase module for two reasons:
- there is a test about the current behavior (so it was the expected behavior since #3867 (closed))
- the purchase uom has not the same value as the sale uom, because when purchasing it is the supplier call to define the uom not really the buyer. More over the purchase uom is just a fallback for when the product supplier does not define one.
In contrary to the sale uom is chosen by the company and this change will allow to compute the sale price for all the products at once with the proper unit.