Paying tax demand on rectified itr2

I had filed e-returns for AY2010-11 and had received a demand notice originally for around INR 2500. I had noticed that there were some calculation errors in the department's computation and in my calculation the demand should have been around INR 850. So I made an additional self-assessment tax payment online for the additional 850 in August 2012 and had filed a rectification request in August 2012 with updated figures for self-assessment tax.

Now I have received the rectified ITR2 but the additional INR 850 I had paid in August 2012 is not allowed in the new computation u/s 154 and the rectified ITR2 has a tax payable demand for INR 1000. Is there some way I can get the additional 850 paid credited against this new demand of 1000 and pay only the remainder 150? What do I need to do to ensure that the 850 is recorded in their systems as amount paid against demand raised?

Thanks for your advise in advance

Satyajit