We changed the transfer license behaviour (july 2019): licenses are now transferred immediately, instead of on renewal date. The new reseller will be invoiced directly.
When a license is tranferred, it stops the license under the old reseller and will start a new license under the new reseller.
Example: a user purchases a yearly license (prepaid), and after 2 months he transfers his license to a new reseller -> that means that a new yearly license will start and that the user will be charged for the new yearly license (and they already got charged for the yearly under the older reseller).
If you want to avoid such a scenario it's best to transfer at the end of their current billing period, and BEFORE their renewal date.