It's a bit confusing, in other sports it's wages that are spread over the contract, like in baseball.
Presumably you could agree to spread the transfer fee over as many years as you wanted, regardless of how long you would actually have the player for. That's what I don't get about this new approach that Chelsea are using. I guess it wouldn't be financially prudent to still be paying for an asset after they had left the business. Transfer fees have been spread over time for ages.
Don't know if I care enough to find out