For many years, Tchaikovsky received an annual income from a wealthy patron named Madame von Meck. What was the single condition to which Tchaikovsky had to agree to receive this compensation?

The single condition wash that they should never meet.

Explanation:

Tchaikovsky thought that he would not live up to Nadezhda Von Mecks expectations, and he also found their relationship awkward. The two of them did not know whether they liked each other or not, and the fact that money was brought up in almost all the letters made Tchaikovsky feel uncomfortable,

They did meet a couple of times, but the meetings were by chance. The first meeting was in a park, were Tchaikovsky was taking a stroll, and Madame Von Meck was on her way home, and the second meeting was in an Opera house.