Is a certified check considered a post-dated check when purchasing dollars with it, in light of the severe shortage of paper currency, knowing that the value of the certified check enters the seller's account and he can make purchases with it?

If a certified check does not have an immediately withdrawable and tradable balance, it is considered a post-dated check, and thus does not constitute a valid legal قبض (seizure/possession) according to Islamic law. For a constructive قبض (possession) to occur via a check, it is a condition that the check has an immediately withdrawable balance.