Roblox games use a client-server relationship. Your cash balance is calculated and stored on the game's secure server, not on your personal computer. A script might change the numbers visualised on your screen (client-side), but as soon as you try to buy a tower, the server checks your real balance and rejects the purchase.
Ironically, even if you find a working infinite money script, it kills the fun. Tower Battles’ core strategic depth comes from balancing early-game farming with defense. Infinite money removes all decision-making—you’ll be bored within 15 minutes. Tower Battles Infinite Money Script