Is the principal entitled to not complete the sale of the land after discovering the possibility of annexing government land to it, which would increase its value? And are his constituents entitled to refuse the sale on the grounds of his incompetence in the agency?

If the questioner has legal guardianship over the agents' money, then his sale is binding on his part and dissolved on the seller's part until the option period expires. It is not permissible to revoke the sale after its conclusion, even if it was not documented, and the occurrence of the mentioned benefit on the land does not affect the ruling. The sale is binding if the buyer ratifies it.