How to organize a transit FTP server?

Question

linux

Colleagues, good afternoon.
There was the following need, sometimes you have to transfer FTP access from a client site to a freelancer. Naturally, we do not know what the freelancer will want to do with this site after completing the task assigned to him.
Therefore, an idea appeared: we provide the freelancer with access to our FTP, when connecting to FTP, the Customer is identified by login and password and the server automatically redirects the freelancer to the Customer's server, maybe through symbolic links or something else. That is, we give the freelancer our access to FTP, which will direct him to the Customer's server. When necessary, we ourselves cut access.

Answer 1

the simplest thing is to forward the tsp-port from yourself to the ftp-port of the client's server ??