What is the ruling of Islamic law concerning fulfilling a promise to pay an additional amount beyond the agreed-upon price of land in a contract, which the heirs' agent secretly stipulated for the completion of the sale?

What we understand from the question is that the questioner is an agent for the heirs in selling their land, and requested an additional amount over its original price for himself without their knowledge.

If this is the case, then he is sinful, because an agent is obligated to act in the best interest of his principal, and it is not permissible for him to take anything from the price for himself without their permission.

This additional amount belongs to all the heirs, even if it is not written in the contract, because what matters is what the two parties secretly agreed upon.

The questioner should advise this agent to fear Allah and endeavor to return the amount to the heirs without informing them of the true circumstances, in order to cover for him and prevent estrangement.