"Provisie" is usually just the attorney's fee / solicitor's fee.

It's not necessarily a retainer. The retainer is something you pay in advance, even if you don't need their services, but the provisie can also be agreed as a flat-fee or a cut/percentage of whatever they collect on your behalf, if a monetary award is sought. In firms that operate on a no-win no-fee basis, for example, that is often the case, but that limits you in scope (i.e. attys will usually only take on cases they are confident they can win or cases where some form of payout can be guaranteed in some other way). In collection attempts (on bills, invoices) or non-performance of payment --- for anything (like a translation contract) --- it's often just a fee they take off the top once they acquire the funds in escrow or in whatever intermediary business account is used to transfer funds between winning/losing parties.

I'd just call it the "fee", because sometimes the fee includes legal costs that they will front for you. A retainer, however, is just to keep the attorney available to work for you whenever you need them to, and make sure they don't work for someone else, especially someone you might be suing.