@THSSC

if I understand this issue, you need to track the duck sponsorships sold per player, and apply that amount to their dues.

the sponsorship item on the invoice should be linked to a liability account you create

each player should be set up as a class, and their sale is tagged with that class

periodically report on the player class, that will show the total sold for each player, then do a player invoice
line one - the dues item, qty is one, and amount
line two - the sponsorship item, qty is negative one, and that players sales amount