At the end of 2022, the United States offered Bulgaria a USD 200 million agreement that needed Sofia to provide its Soviet air defense systems to Ukraine, "including S-300 missile systems," Defence Romania reports with reference to 24 Chasa.
The funds were supposed to be allocated as part of the Foreign Military Financing (FMF) program which is also used as compensation for supplies of various defense materiel to the Ukrainian Army provided by European partners.
In the case of Bulgaria, unofficial reports say American authorities proposed to send modern NASAMS air defense systems as replacements for the S-300. However, back then the acting Prime Minister of Bulgaria Galab Donev remained silent as to the US offer, which made the process drag from the get-go.
Then, in January 2023, Bulgarian President Rumen Radev and acting Minister of Defense Dimitar Stoyanov publicly opposed any further weapon supplies to Ukraine, even if that happened according to the "ring exchange" procedure. The said reason was, Bulgaria's national law prohibits giving or selling any of the Bulgarian Army's arsenal without an immediate compensatory replacement.
Earlier Defense Express reported that Ukraine filed a request to Bulgaria asking for S-300 after unsuccessful pleading for MiG-29 and Su-25 combat aircraft.
According to open source data, the Armed Forces of Bulgaria have approximately eight S-300PMU systems, produced and supplied in 1989 which is relatively "fresh" compared to the other systems of this type. That, as well as long-range capabilities and familiarity of this system to the Ukrainian Armed Forces, makes the Bulgarian S-300 very valuable and necessary for defending the airspace of Ukraine from russian missile attacks.
Read more: Take Chassis From Germans and NASAMS Elements from Norwegians, and You'll Get a SHORAD for IRIS-T or Even AIM-9X Missiles