Problem solved!
It turned out that I had accidentally entered my son's net income as $866,000 instead of $8,660. That info is entered way before any tuition transfer stuff, and I didn't connect the two. As soon as I corrected his income, the amount of tuition transferred was automatically set to $5000.
It would have been useful to have the message say something about the reason I couldn't enter more than $0.
Thanks for the help.
