Is it permissible to mortgage a partner's share in land if the loan obtained by the remaining partners involves usury (riba), knowing that the partner does not benefit from the loan?

You are not permitted to sign for a usurious loan or mortgage your share in it, because that is cooperating in sin. Allah Almighty has forbidden cooperation in sin and aggression. The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes Riba (usury), the one who feeds it, the one who writes it down, and its two witnesses. So, whoever assists in falsehood is a partner in it.