13 If the owner wishes to redeem the animal, a fifth must be added to its value.

Read Leviticus 27:13 Using Other Translations

But if he will at all redeem it, then he shall add a fifth part thereof unto thy estimation.
But if he wishes to redeem it, he shall add a fifth to the valuation.
If you want to buy back the animal, you must pay the value set by the priest, plus 20 percent.