Commentary for Arakhin 7:2

ואחד כל אדם – who redeems a Bet Kor/an area requiring a Kor of seed, whether it is worth one-thousand shekel or whether it is not worth anything other than fifty shekel.

It is all the same whether the owner or anyone else [redeems the field].
What is the difference between the owner and any other man? The owner must add one fifth, whereas any other man need not add one fifth.
The reckoning of the value of the field is basically the same whether the owner or another person redeems it. The only difference is that the owner adds another fifth to the value of the field. This is stated in Leviticus 27:19: “And if he who consecrated the land wishes to redeem it, he must add one-fifth to the sum at which it was assessed, and it shall pass to him.” The mishnah reads the words “and if he who consecrated the land” to mean that only if the owner consecrates it must he add another fifth. If others do, there is no added fifth.

שהבעלים נותנים חומש – as it is written (Leviticus 27:19): “and if he who consecrated the land wishes to redeem itג, [he must add one-fifth to the sum at which it was assessed, and it will pass to him].”