The following is an alleged letter sent by Banda during his struggle against the Mughals:
J.N. Sarkar’s transcripts of the Jaipur Kapaddwara records contain a letter that may be from Banda to Jai Singh of Amber. The letter begins with Banda’s characteristic salutation ‘Fateh darshan and expresses the writer’s surprise at Jai Singh s having ‘forsaken Hindu dharma, especially since ‘Akal Purakh has commanded that the time of the Turks has reached its end and it is the turn of the Hindus.’ The letter states that ‘the foundation of dharma must be strengthened’ and asks Jai Singh to join the writer in marching on Delhi ‘to kill the oppressive Turks.’ It ends by stating that ‘I do not need sovereignty (shahi) but have been sent by Akal Purakh to strengthen the foundations of dharma (National Library, Calcutta, Sarkar transcripts, notebook 105, p. 2). The exact nature of the letter is problematic, especially as the original is not listed in the latest catalogue of the Kapaddwara collection (Bahura and Singh 1988). The grammar and diction are occasionally irregular, and the transcription bears the name “Guru Gobind’ at the bottom, although it is unclear whether the words are an addition by Sarkar or by a Jaipur scribe or record-keeper. It is of course also possible that the letter represents a transcript of a forged letter sent to Jai Singh in the name of Banda.
Source: Sikh Religion, Culture and Ethnicity by Christopher Shackle, Gurharpal Singh and Arvind-pal Singh Mandair. Curzon Press, Richmond, Surrey, pg’s 44-45.
dharmayuddha 
Any proof of a reply as well? That might say something about the authenticity of the letter.
Karmasura not that I know of but contemporary Mughal chroniclers do refer to the fact that Banda was in touch with various Rajput rulers, so we do know that there was communication for sure. If I find anything more I will update it here.
Thanks for the comment.
The posts on Sivaji’s deterrent actions against missionaries are very interesting and useful. Thanks.