I don't know if Samsung can accept a trade-in for a device you haven't fulfilled your agreement with to own. Have you tried seeing if the online system will even accept it? When Samsung gets your trade-in, that is when they will check the device to see if it is free and clear, lost or stolen, bkacklisted, erc. This might invalidate your whole trade-in offer with them. In my personal opinion, you should fully pay off your Z Flip6 first or just buy the s25 Ultra outright without trading in your Z Flip6.
