Burdokva,
Pilot Ivan Bonev is mentioned in this article
http://odin.prohosting.com/~vstoyano/Stoyanov%20Me109.htm
where he's credited with a shared B-24 on 1 August 1943.
The article itself seems an interesting read, and not only for Bonev.
In the book "In the Skies of Europe" by Hans Werner Neulen a reference is made to what you wrote: "
Once again, there was an unpleasant incident on 11 June (1944), when a Bulgarian pilot was deliberately killed by American fighters while hanging helpless beneath his parachute."
Would this have been Bonev?
Regards,
Leendert