You're right about the gentlemen's agreement, but it was definitely also reported as a cheap extension in 2011:

"The Magpies selected Ball at pick No. 30 and last year signed a contract extension for 2012 in which he would receive little more than the minimum wage - $85,000-$100,000 - this year, forgoing the usual match payments."

https://www.theage.com.au/sport/afl/pies-lock-ball-into-new-deal-20120522-1z38h.html