I think it depends entirely on what is written on the contract. If you wrote on the contract that you would pay her $4000 for example, no matter how poorly she performs, it would be illegal to pay her less than that (ie, YOU would be breaking the contract). If you wrote that you were paying her $3700 and actually gave her $4000 each month, then that would be considered a "bonus" and you legally could forego the extra $300 for underperformance. It all depends on what is written in the contract.
