Reconveyance deed form no7,

Sir, Iam mohan, my father was in govt job, he has taken house building advance of 1,80,000 from pwd department and as a security they get our plot mirtgage (mortgage deed), it was the same plot for which we applied to house loan. But he died in 2011 at the age of 52 without fully repaying it. So rest of the sum was deducted from gratuty and other insurance amount along with interest, after that we got NOC from pwd and pension get started to my mumy. But now they gave us a form Reconveyance form no 7, and wrote my father's name instead of my mumy, and ordered us to get it register at sub registrar bagpat. But registrar refuse it to register cuz my father is not alive. He also asked us to made reconveyance form to my mumy name but department refuses and ask us to give legal heir sertificate to department to made reconvayance deed to my mumy. We made a vaarisan praman patra from district megistrate n gave it, but they refuse it and says it has one line ( ?? ???????? ???? 5000 ?? ?? ??? ????? ??) so it is not valid. Then department ask us to provide succession Certificate. I asked in district bagpat, some says it is made only on property and your property is mortgaged so u cant apply for it. Please tell us how can we prove our legal heir certificate to them.