What is the ruling on paying the old fare to bus drivers who are unaware of the fare increase, and are they indebted to us for the fare difference, and how can we return their money to them if that is the case?

There is no obligation to pay the difference between the old and new prices. The determining factor for the fare is what was agreed upon in the contract. The ignorance of the bus owners regarding the increase does not affect the validity of the contract. The Prophet's (peace be upon him) prohibition of a resident selling for a Bedouin is evidence that a contracting party's ignorance of the price does not affect the validity of the contract.