John Wesley on Deuteronomy 25:9

Loose his shoe - As a sign of his resignation of all his right to the woman, and to her husband's inheritance: for as the shoe was a sign of one's power and right, Psa 60:8 108:9, so the parting with the shoe was a token of the alienation of such right; and as a note of infamy, to signify that by this disingenuous action he was unworthy to be amongst free - men, and fit to be reduced to the condition of the meanest servants, who used to go barefoot, Isa 20:2,4.