Due to the shortage of S-400 SAM systems as well as the loss of their components on the battlefield as a result of the successful actions of the Ukrainian Defense Forces, They in russia are confident that Ankara is unlikely to be able to use these systems on the battlefield, while the missiles to them will soon require maintenance - so it may be interesting for Turkey to simply sell them back to the russians.
Read more: Ukraine Constantly Develops New Methods Against russian Electronic Warfare and Air Defenses

The Nefes publication writes about this possibility. However, there are still many questions in this story. It is also currently unknown under what conditions russia will agree to buy back the SAM systems delivered earlier to the Turks.
From the Defense Express side, first of all, let us recall that Turkey ordered its four S-400 SAM systems in 2017, paying $2.5 billion for them. It is unknown how much Russia currently values these air defense systems in order to buy them back.
On the one hand, this could be a much larger amount given the global trend of rising arms prices. On the other hand, Turkey is unlikely to ever be able to fully use its S-400s, so russia may use this fact in potential negotiations to pressure the Turkish side in order to get the most favorable conditions for the probable deal.
At the same time, Turkey also has "trump cards" in its hands, because it is precisely the russians who urgently need to quickly obtain additional S-400 SAM systems. Given this, Ankara may well ask for a high price for these systems, and use the funds received for its own 5th-generation fighter project.

It is worth recalling that the United States generally criticized Ankara's purchase of russian air defense systems, in particular, given that Turkey is a NATO member state. Immediately after receiving the first S-400 SAM systems, Turkey was excluded from the American 5th-generation fighter program F-35 literally a week later.
All this happened during the first presidency of Donald Trump. Given this, it can be cautiously assumed that today the most important thing is how the United States will react if Turkey responds positively to the russian proposal. This deal is bound to fall apart before it even starts, given the likely critical backlash from the US and Turkey’s previously announced desire to return to the 5th-generation F-35 fighter jet program, as well as plans to purchase an additional batch of 32 F-16C Block 70s.
In practice, it is not out of the question that Trump and the United States might find it acceptable for Turkey to get rid of its S-400s and sell them back to the Russians. This would, of course, be a very negative scenario for Ukraine, as Russia would receive additional S-400s in a short period of time, significantly enhancing its air defense capabilities.

As Defense Express reported, Ukraine undermined russian air defenses in Crimea, forcing russia to divert resources. We also wrote, that Ukrainian UAV Forces continued successful hunt for russian air defenses.
Read more: What Ukrainian Drones with 15 kg Warheads Can Do and Why That’s Enough to Cripple russian Air Defenses in Crimea