Is it permissible for an accountant to sell foodstuffs to a manager in his company for less than the market price during official working hours without his sponsor's knowledge?

There is no objection to a person working for himself with someone other than his sponsor outside of the agreed-upon working hours, unless the sponsor stipulates that he not work for anyone else, even during his free time. In that case, he must fulfill the condition. He is not permitted to work for himself during official working hours except with the sponsor's permission. If custom prohibits someone in the questioner's line of work from engaging in the same activity for himself with his company's clients, then he is not allowed to do so except with his sponsor's permission. This also applies to his colleague.