In vanilla minecraft with commands or as a spigot plugin? I don't know about commands, but it would be pretty easy to do as a plugin by adding a handler for the
EntityDeathEvent that checks if the entity was that cow and then clears the drop list and adds 1 leather to it.
Edit:
I noticed that if you put an item in a cows hand, it will not be visible but you can set the drop chance.
This summons a cow that drops an extra leather:
Code:
/summon minecraft:cow ~0 ~1 ~0 {HandItems:[{Count:1,id:leather},{}],HandDropChances:[2.0F,0.0F]}
I don't know how to disable the regular mob loot though.