Are funds acquired from selling dollars using certified checks – where an agreement is made on one day and delivery occurs two or more days later due to the bank's delay in issuing the check – considered unlawful (haram) and must be disposed of, knowing that I did this believing it was permissible and stopped immediately upon learning it was impermissible?

As long as a person believed a transaction to be permissible, then later learned of its prohibition and ceased engaging in it, there is no sin upon him for what he acquired before that; due to the Almighty's saying: "So whoever receives an admonition from his Lord and stops, then he will have what is past, and his affair is [left] to Allah. But whoever returns [to interest] - those are the companions of the Fire; they will abide eternally therein." (Al-Baqarah: 275).