Leviticus 27:31

31 Whoever would redeem any of their tithe must add a fifth of the value to it.

Read Leviticus 27:31 Using Other Translations

And if a man will at all redeem ought of his tithes, he shall add thereto the fifth part thereof.
If a man wishes to redeem some of his tithe, he shall add a fifth to it.
If you want to buy back the LORD ’s tenth of the grain or fruit, you must pay its value, plus 20 percent.