Is it permissible, Islamically, to deduct the remaining amount (20,0 SAR) or part of it, due to fraud in the quantity and poor quality of the cattle herd's production, after having paid 40,0 SAR out of the agreed-upon 60,0 SAR?

If the buyer stipulated that the cows must produce a certain number of pounds of milk, the sale is invalid due to manifest gharar (definition) (excessive uncertainty). Both of you must return what you took to its owner, and then you can conclude a new contract. However, if there was no stipulation but the seller misrepresented their milk production, the sale is valid, and the buyer has the option to return the sold item or keep it. You are not permitted to refuse to pay the remaining price.