John Gill Commentary Leviticus 27:31

"And if a man will redeem aught of his tithe, he shall add unto it the fifth part thereof." — Leviticus 27:31 (ASV)

And if a man, will redeem [ought] of his tithes
Of his own, and not his neighbour's, as Jarchi observes; for if he redeemed the tithes of his neighbour, but did not add a fifth part, which he was obliged to do if he redeemed his own, as follows:

he shall add thereunto the fifth [part] thereof ;
besides giving the value for what part of his tithes he redeemed, he gave a fifth part of that sum over and above; as, supposing the tithe was worth fifty shillings, then he gave that, and ten shillings more, and so in proportion. The use of this redemption, as Jarchi suggests, was, that he might have liberty of eating it in any place: for he understands it of the second tithe, as before observed, and which was to be eaten at Jerusalem.