From the rules here
The reserved cards are kept in hand and cannot be discarded.
Players may not have more than three reserved cards in hand,
and the only way to get rid of a card is to buy it (see below).
To purchase a card, a player must spend the number of
tokens indicated on the card. A joker token can replace any
color. The spent tokens (including any jokers) are returned to
the middle of the table.
A player may purchase one of the face-up development
cards in the middle of the table or a card in his hand that was
reserved on a previous turn.
It's entirely legitimate to reserve a card another player might want as long as they don't exceed the 3 card hand limit.
The rules do NOT say they gold token has to be used to purchase the reserved card. The rules make it clear a reserved card can not be discarded. The rules state nothing rules about it returning to the stacks at any point.