This is a good question, and it requires a good answer--even though I think it is a very big (general) answer, to a very big (general) question.
I'm not a Bible scholar, so I'm not going to insert proof texts, but I think one could find dozens there...along with knowledge that God's people understood God declaring something true now...when to them it really was not yet, as part of daily existence.
First of all we have to ask how does God see and know what is true. God is eternal, and sees things eternally. Jesus' blood paid the price of sin for everyone who believes...that's an orthodox statement, something that can be accepted by Roman, Protestant, and Eastern Christians alike. Meaning 2000 years ago at the cross--everyone's sins, past (the OT saints) present (those fearful and then unfaithful disciples), and everyone who's believed and followed Christ since--had his sin completely washed away at the cross. So before we ever were born...or sinned at all...it was paid for. Strange, and hard to ponder, I agree, but this is what Holy Scripture teaches, and what orthodox Christians have always taught. I'm giving this fundamental example to show the timelessness of God...His work although effective in our time, is really beyond our time too.
Now why all this philosophical gobblygook, to try and show in Scripture "where... does God EVER declare anything which is not either already true or is simultaneously accomplished by the very act of God making that declaration?"
Very simple:
The Old Testament sacrificial system.
Think about it, orthodox theology has always held (until some way out, admittedly Protestant, dispensationalists denied it) that the Bible teaches Jesus' sacrifice was what the lambs, bulls, doves, and goats sacrificed for sin, and peace offerings, had always looked forward to...but not accomplished, EVEN THOUGH GOD CLEARLY DECLARED FORGIVENESS THROUGH THOSE O.T. ANIMAL SACRIFICES.
Those repeated sacrifices...every day...and on some days by the hundreds...were not enough, in and of themselves, to cleanse from sin. If Jesus had not come, and completed the Law--including the sacrifice, in His own blood, those O.T. saints never could have seen Heaven, as their faith, in the animal sacrifices, was really looking for God's provision--which was, though they did not know it, in Christ Jesus' life and work on the cross.
So logically, every time in the O.T. when God forgave them...through the sacrifice, offered after genuine repentance, God was declaring something as true, which was NOT already true, and not made true by His declaration. From the O.T. time standpoint, forgiveness was not a fact yet...YET is the big word. But also it was true they had been forgiven, as God would in the future surely do what He had promised--from Gen. 3:15 (speaking to the devil)"it shall bruise thy head, and thou shalt bruise his heel," onwards.
So the whole of the O.T. sacrifices can be cited as places in Scripture where God declared something true, which wasn't yet...(but would be, based on His integrity) nor was it accomplished by fiat at His word.
In the same way every Christian is seen by God as having the perfection of Christ (as we will, someday--and in a sense as God sees it NOW in His time) even though, from our finite in-time perspective, we are all far from perfect and complete. God loves us, just as He loves Jesus, and how in character, united in Him, we will someday be.
Jesus paid it all.
And Amen!!!