How do I do something in tower defense

So the thing that I can figure out is, I have an instance variable on my tower and I am able to build my tower without any problems. The thing that I am stuck on is when the money gets up to $30, which is the amount it costs to build the tower, and I select the already built tower, it will delete it and take away $30 from the funds available. I have tried a few things and I just can't seem to wrap my head around it. Any help is greatly appreciated and I hope I worded it right for you guys to understand. So one more time, tower is built and it cost $30 to build. when I have $30 again in the bank and I select the tower it will delete the tower and deplete $30 from my bank.

are you building this from the tower defense template?

there is a rule.. it looks like it's line 15.. if you place a turret on a location that already has a turret, destroy them both.. you might want to adjust that rules to not do this.. or handle it differently