Transfer add-ons are an extremely useful feature in the game, they can let you get a player for cheaper than you normally would, but with a few little strings attached, that all-in-all, will hardly effect you.
This guide will show you how to utilise these to your advantage, and what they will do.
 Additional Clauses
 Percentage of Next Sale
This option, in short, means that you give the club you are trying to buy a player from a certain percentage of the money you receive when you next sell him.
This can be used well if you're buying a player over the age of 28/29, as it's fairly unlikely you are going to sell him. But when you're buying a younger player, it's better to try and avoid this clause, as at some point in the future an offer may come in that your board just can't refuse, and then you only got 50%(or whatever percentage you set) of what you sold him for.
 Buy Back Price
This clause allows the club you are trying to buy a player from the option to purchase the player back at any time for a certain price.
I don't see any use in this clause, but it can be useful when you are selling a player.
 Arrange Friendly
This clause is simple really, most smaller clubs want this option included if you are a big name team, as it can fetch them a lot of money.
I see no harm in allowing this one, as I think you can actually cancel the friendly once the transfer has gone through, thought I'm not certain.
 Additional Fees
 Monthly Installments
This fee means that you will pay a certain amount of money to the club over a period of 6, 12, 18, 24, 30, 36, 42 or 48 months.
This fee isn't taken from your transfer funds, but your overall balance, which can be useful if you have a large balance, but you should steer clear if you've got a balance under, say, £15m.
 After League Appearances
This clause means that after the player you are trying to buy has played a certain amount of games for your club in the league, they will be paid a certain amount.
It's a pretty good feature, and can be used to hold off paying a bigger sum, though it too comes out of your bank balance.
 Per League Appearance
This is similar to the 'After League Appearances' clause, but instead of paying after a certain amount of games, you pay per game.
It's not that useful in my opinion, it's much easier to just use the 'After League Appearances' clause.
 After International Appearances
This is exactly the same thing as the 'After League Appearances' clause, except it's for the international side of things.
It can be used to good effect, especially if the player is from a big country, like Brazil, or Italy, as there's a smaller chance of them being picked for the squad.
 Minimum League Goals
This clause means that you will pay the club you are trying to buy a player from a certain amount of money once he scores a certain amount of goals.
It's a good thing in theory, but with some of the ridiculous goal rates players get, it's a bit risky to put in a big sum of money on this clause.
 After Promotion
This clause means that you pay the club you are trying to buy a player from a certain amount of money once you get promoted, now obviously this clause will only work if you're in the league below the top division of that country.
Unless you know what sort of money you'll get the next season, and if you're promoted, then it's best not to use this, as it can land you in a bit of debt.
 Players To Exchange
 Transfer Listed
This clause lets you select a player who is on your transfer list to be put in as part of the deal when buying another player.
It's a great thing in theory, but other clubs will rarely want the player you've included, so it's a rather useless clause.
This is the same as the 'Transfer Listed' function, except that it let's you choose someone who isn't transfer listed, and from the rest of your squad.
It's no different to the 'Transfer Listed' option, and it's hard to get a player included into a deal, unless your assistant manager tells you that the club has been interested in signing one of your players.
Credit to Super Spurs for the guide