The definition of rescue:
Save (someone) from a dangerous or distressing situation.
An act of saving or being saved from danger or distress
Rescuing a betta (or any other animal that otherwise would have died) is rescue, regardless of whether you paid for him/her.
The only problem with paying for a betta that you have "rescued" is that it supports the abuse these lttle guys suffer at petstores.
However, even if you buy a healthy betta, you are supporting this.
And leaving a sick/dieing betta there does not make the store loose money, they are compensated for that lost normally
If you find a betta in a bad situation at a petstore, the best option is to find a manager and ask that you get a discount, if you are calm and polite, explain what is wrong with the fish and how it is going to cost you a lot to save himher (never hurts to mention you will have to buy a lot of stuff from said place to save him/her) the manager is almost ALWAYS willing to give you a discount or even a free fish
*I got my fish Hercules for free after he was placed in another bettas cup, he was torn up really bad
*I got Finn (a CT betta) for the price of a VT, he has NO fins left, so not the best deal, I really shoul have gotten him for free but I wasnt in the mood for arguing since I wanted to get him home