The actual rule for my post above (page 4):
If you already have 3 incomplete commissions and you want to buy another, you must first return one of the ones you already own. To accomplish this, place the card onto the commission book for the city you are in. You may decide whether to return the card to the right or left side of the book.
And, the rule for Michael's post, page 5:
You may choose to unload a gemstone that you had previously loaded onto a horse or cart, in order to reload a different gemstone. Place the unloaded gemstone onto the discard space in the middle of the gameboard. To load the new gemstone, you must again pay the cost in coins shown on the card.