This was always the problem with the 18 month loan with option. If Sands' value increases over that season-and-a-half, then Rangers can trigger the clause whether or not they intend to keep him - they can buy and then sell on at a profit. If his value does not increase, then we get him back, but at a time when we have secured replacements and would prefer to sell him. So, we likely sell for less.
That said, there were a few MLS players sent out on 18-month deals. It's not clear to me why, but it's something that was common in the market last off-season.