To many slaves the paying, and the taking of arrears isn’t enough.
In those cases the words “borrower’s note”, “contract” or “blackmailing” appear. The borrower’s note is an official form which binds the slave to a monthly payment. Many mistresses think that they simply can write something in word, send it to their slave and bind him – but thats a fail-guess because most of those “Contracts” are not judicial correct.
There are many original borrower’s notes available – but a real one always requires mistress and slaves full name, adress and birthday and of course a signature. Since the anonymity is getting lost in this case, you have to take care.
My tip: No contracts before knowing your slave less than 1 month.
Blackmailing is the other level. The slave fills out a form where he tells everything about him and his family, friends and co-workers – mostly also a secret, to make himself vulnerable and blackmail-able. Blackmailing is forbidden and I advise caution! Many slaves do not know what they are doing while they fill out those forms in a sexual-flush.

And after they did it, they are the most unhappy people on earth. And as I said, its accusable and criminal. No matter what slave asks you for it, I recommend you not to accede.