Hello Chintan,
You should have given all these inputs in your first posting itself, else you would have run the risk of being misunderstood. Coming to your issue, there are a few points I would like to highlight, basically looking at it from different perspectives.
1) Though your interest lies in HR, did you ever realize that you do have a flair in marketing - hadn't it been for your boss's decision? And as you said, you seem to have been effective too, the reason why he didn't want to relieve you.
2) We will always be happy with those WHO WANT US, rather than WHOM WE WANT...this is true in career or life per se, except that you need to be clear on the REASONS why the other person/company wants you - there could be cases where the other side may want to just use you. But based on what you mentioned, I think the reasons why your ex-employer wanted you were: ONE he really trusted you & TWO he was impressed with your performance.
3) In the present situation of the recession, I wouldn't blame him for moving you to marketing, especially when half the people left. In a line, HE WAS BANKING ON YOU to tide over the present tough times. You should have discussed this issue openly with him, telling him that due to the situation you can handle marketing but would like to move back to HR as soon as the situation improves.
4) I am not sure of the domain that company was in, but unless it's something in a totally down-the-tube scenario in that domain - meaning no chance of improving even after the recession is over, financial problems are pretty common all over the industry today. Did you give a thought to how your treatment would be after the recession/improving of finances? I will bet my last penny on this - it will be great and you could be one of the trusted guys in the whole company, with added responsibilities.
5) You need to have BOTH short-term/immediate goals AS WELL AS long-term goals in life. I won't say I am right, but I do have a hunch that this opportunity that came your way COULD be aiding your long-term goals in some way, where marketing exposure could be a major plus for you. Many times it happens that way Chintan, while we are in the situation, we think: what's the hell is happening. But only later do we realize the 'why' part. Quite a few times, the chance is missed.
Now coming to the question: what to do now? If this ex-boss was a man of word, basically meaning 'doesn't use people' [you will be the best judge on this], maybe you could consider going back to him and discussing your apprehensions threadbare. Be as plain as you can - after all, everyone makes mistakes, but the ones who don't hesitate to correct them are the ones who go far in life. At least, give it a shot. The max what could happen is: he will say 'forget it'. I don't want to take you again [this could be possible if you had heated arguments in the recent past]. At least you did what you could. And if you aren't welcome there, obviously you need to be prepared for the consequences for what you did, however well-meant or unknowingly.
And never ever quit a job, however lousy or whatever the situation...BEFORE YOU HAVE ANOTHER IN HAND - easier said than done, since I know HR guys doing it too - which is surprising.
Regards, TS