Once a person has been sentenced to die (which only has happened once in modern Israel), he cannot be redeemed for money because his life is not worth anything.
The tithe of the land belongs to the Lord so if someone redeems it they have to add a fifth to it. They might want to redeem it to give it to a foreign land.
I suppose here the priests are looking out for their reserves.
When you give your tithe in lambs or calves, then you should stand by the gate and hit every tenth one with stick that has been dipped in vermillion.
You should not check to see if the animals chosen for the tithing are good or bad. But if you do, you shall not offer a substitute for it. And if you do, then both the “one and its replacement are holy” ... in other words, you need to give the good one and eat the bad one. You should not use the bad one for shorn (wool) or labor. This is all about being fair, isn't it?