What is the ruling on selling cabbage/head of cabbage by number rather than by weight or measure, given the variation in their weights and the fixed price per unit, and what should the buyer do about this, knowing that this is the common practice?

It is permissible to sell cabbage by count at the customary price, such as dirhams, dinars, and paper currency. There is no objection to the variation or equality of the prices of individual cabbages, as this is up to the seller. There is no issue for the buyer in dealing with a seller who sells cabbage by count without considering its weight.