Hi Johann,
Great news; such a thing exists! https://marketplace.whmcs.com/product/4993
We do not have plans to create a generic or sub-reseller cPanel license reseller module, in part because cPanel's Manage2 API doesn't have the concept of tracking or authenticating sub-resellers such as yourself.

In the immediate term, what they could do is switch to the fixed price (non-auto-scale) cPanel licenses.
That way you and your clients have a predictable regular price. Clients can login to their client area to place an upgrade order when they want to move up/down a cPanel license tier, and you can then update this with your reseller manually.

In the longer term, provided your license reseller has an API which exposes the number of accounts associated with a cPanel license, you could create a provisioning module which leverages the Usage Metrics feature to invoice clients based upon the number of users: https://developers.whmcs.com/provisioning-modules/usage-metrics/