If you pay attention and do the math you will notice that somehow more units were purchased than I had money to purchase. 3 subs + 9 infantry comes to $150 but I only had $133. The money for the extra sub seems to have come out of the money that I received at the end of the turn so in essence I purchased a sub on credit and then had the money for it taken out of my next paycheck.
Also, the extra sub was not actually purchased. The system purchased it for me. I only wanted 2 and I only moved 2 from the tray to the board.