If by the end of week 12 you have traded away ALL of a future years draft picks, you must pay for that year in full ($40) or owner rights will be suspended. If you traded away more than half of your picks (3 out of 5), you will be required to pay half of that year ($20). This rule is put in placed to keep owners from leaving the league. It also helps fill a team if the team bcomes an orphan. (For example, if I still have all my 2019 rookie picks, but traded away 3 out of 5 of my 2020 rookie picks, I need to pay $20 for 2020)
You are going to run into issues with owners leaving after trading away picks. New potential owners are less likely to take a team if its future has been traded. I highly suggest making them pay the full entry fee (non refundable) prior to trading a single future pick. Paying a full entry fee hasnt stopped owners from leaving anyways at least not in my leagues.