He got a certified cheque in order to buy a house and one year has passed; does he have to pay zakaah?
If you gave the owner a certified cheque and the money indicated by the cheque was no longer under your control, then you do not have to pay zakaah because the money was no longer in your possession. If it was possible to cancel it and dispose of the money, such as if the cheque was still in your hands or the sale of the house had not yet been completed, then zakaah became obligatory on you when one year had passed, because if the cheque was still in your hand then you were still in possession of the money. If the sale had not been completed but we assume that you gave the cheque to the owner then it is regarded as a debt that was owed until the sale was completed, and it is obligatory to pay zakaah on debts that one is owed by other people.
You did not state whether you gave the cheque to the owner or not, or whether the sale had been completed and it only remained to hand over their house or it had not yet been completed. If you are uncertain about anything in this answer, then you may ask again with more details.
And Allah knows best.