https://legal-dictionary.thefreedictionary.com/Attorney's Li...
Attorney's Lien
The right of a lawyer to hold a client's property or money until payment has been made for legal aid and advice given.
In general, a lien is a security interest used by a creditor to ensure payment by a debtor for money owed. Since an attorney is entitled to payment for services performed, the attorney has a claim on a client's property until compensation is duly made.

https://definitions.uslegal.com/m/medical-lien/
Medical Lien Law and Legal Definition
Medical Lien is the right of a health care provider, doctor, or hospital to assert an interest in personal injury recoveries of its patients.

https://www.europe-lawyer.com/fr/honoraires/
provision (à valoir) sur honoraires d'avocat

http://residencechateau.fr/chambres_tarifs_leChateau.html
provision sur frais médicaux

(plus rare en France)

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Note added at 9 hrs (2019-11-07 21:43:02 GMT)
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S'agissant de "copies", il faudrait donc tourner la phrase de façon à parler de documents ou de pièces signées - par exemple :
"Veuillez fournir les copies des attestations de provision (à valoir) sur honoraires d'avocat ou frais médicaux, ou de toute autre garantie que vous avez signée."