Is it permissible to keep the difference in the estimated customs revenues that exceeds the amount actually paid, with the intention of paying it to the state in the future if requested?

You are not permitted to keep the surplus from customs taxes beyond what you have paid. Rather, it must be returned to the mandating company, unless the company authorizes you to own it. Taking the surplus without permission is considered deception and betrayal, as Allah Almighty says: {And do not transgress. Indeed, Allah does not like transgressors.} (Quran 2:190). Al-Shafi'i, Al-Buhuti, and Ibn Nujaym have all stipulated this. Evidence for this is the hadith (definition) of Urwah ibn al-Ja'd al-Bariqi, who bought two sheep for a dinar and sold one of them for a dinar, then returned the dinar and the sheep to the Prophet, peace and blessings be upon him. Had the surplus been his right, he would have kept it. "If there is a surplus, the surplus belongs to the mandator, not to his agent who did not act justly."