How can a farmer, knowing the harm, soothe his conscience after using a pesticide on citrus fruits before the recommended harvest period, fearing he may have caused harm to consumers?

You erred by violating the established practice of expediting medical treatment and by asserting that a small amount of medication would not harm, without consulting specialists. You must repent to Allah for endangering people's health. You are not obligated to guarantee anything in the absence of certainty regarding the injury.