I'm DMing using the "Buying a Magic Item" downtime activity rules in Xanathar's Guide to Everything (p. 126). Are any rules (or suggested approaches) to allowing my players to purchase one of their discovered magic items at a later date?
For instance, one of the players rolled for a Saddle of the Cavalier but none of the party has a horse. Do the rules allow the player to track down the merchant at a later date to buy the item? Or is this more of a DM's discretionary decision?