Is the conversion of a post-dated check into another currency considered Riba Nasiah (usury of delay) if the exchange rate on the day of deposit is announced, even though the payment is delayed and the exchange rate might differ?

The question is not clear enough to make a judgment. However, selling a post-dated check for cash is considered riba an-nasi’ah (interest due to delay) because one of the two countervalues is delayed and not immediately seized. The noble hadith states: "Gold for gold, silver for silver, hand to hand, like for like, and any excess is riba. But if these classes differ, then sell as you wish, provided it is hand to hand." Currencies take the place of the cash (gold and silver) in their rulings. Seizure can be physical or constructive, such as bank transfers or certified checks. As for uncertified and post-dated checks, they are not considered constructive seizure.