The Ran (Nedarim 85a) cites the Rashba’s psak that if a person takes a neder not to benefit from his/her own property, others are permitted to help themselves to the nodeir’s goods. Since the property is assur b’hana’ah to the one who took the neder, it has no value; the nodeir suffers no loss. However, if the nodeir was to be matir neder, the neder is retroactively considered non-existent (chacham okeir neder m’ikaro), and those who took the property of the nodeir must now pay for the goods.
R’ Shimon Shkop (Sha’arei Yosher I:9) writes that this Rashba is a clincher proof to the chakira (raised by R' Elchanan) whether issur hana’ah is a function of not being halachically allowed to use something or the item practically having no value. If issur hana’ah is a function of whether something practically can be used, the fact that a neder is considered uprooted retroactively is irrelevant – the practical bottom line remains the same that the item was off limits until the chacham was matir neder. The Rashba only makes sense if one assumes that issur hana’ah is a function of legal status, of the halachic permissibility to use or not use an item. Once the chacham uproots the neder, the legal status changes retroactively. Therefore the items taken from the nodeir are retroactively considered valuable, and must be paid for.