How to define the QUOTA/Target on number of Items sold in a month.

 Hi All

I have a requirement in which we need to fix the monthly car sale TARGET for sales rep and if the target is achieved then pay the commission on that. Also we need to put the check that incase some car is returned then that too taken into account and the total cars should should be = Total Cars sold in that period - any return cars in that period.

And if this number is greater than or equal to 10 then pay the commission on all the cars sold. Else no commission. Pls advise as how to implement this funcitonality.