Negotiations between Tito's partisans and Germans in 1943 are well known.
"Đilas and Velebit were passed through the German lines to Sarajevo and were then flown to Zagreb on 25 March in a military aircraft."
Interesting fact that British who received main part of strategic intelligence from ULTRA operation probably knew this very well, but were silent, and Churchill pressed for recognition of Tito as sole ally in Yugoslavia which culminated on switching Allied support for him on Tehran Conference 1943. Why were British intelligence community ignorant about fact that main ally in Yugoslavia is secretly negotiating with Nazi Germans in 1943? Was it 'I do not care' attitude or Churchill struck some deal with Stalin to leave Tito as only leader of resistance and postwar order in Yugoslavia? Strange fact for me - you know very well that top Yugoslav partisan people are negotiating secretly with Germans and then three months later you read some top intelligence digest and came to conclusion that "it gave a full account of the marvellous resistance by the followers of Tito and the powerful cold-blooded manoeuvres of Mihailović in Serbia.".