Below is the video clip in Farsi with English subtitles where Khamenei explains why he rejected this offer.
From his body language it seems to me that he is under a lot of pressure.
These are not easy days for the supreme leader of the Iranian regime.
Ahmadinejad and Larijani are tearing each other apart, the economy is losing millions a day because of sanctions while cronyism is increasing.
Khamenei has to balance about the 10 different things at the same time. Then there is also the question of his ideology and historical view point towards the US. All will be explained in due course.
Meanwhile here is the clip: