Minion Hiring Bug (in General)

I was hiring a minion , when i did not have enough cash for the big one. I went to click the lowe costing one , but then my hand hit enter , since i am on a laptop , and i bought the bigger one . I went into negative cash , and I told shade about it. We had a little laugh , and then he told me that it shouldn't have let me get the bigger one if i did not have the cash . Is this a bug or just how it works?