Spanish death duties

Hi, I am a resident in Mijas Costa, Costa del Sol, my husband died 3 years ago in Spain and my lawyer completed the paperwork for the death duties. He told me I was below the €175,000 tax threshold and therefore no taxes were payable, just the legal fees. One year later I had a phone call from my lawyer to say I was being chased for €31,000in death duties, I was told that they believed I was not resident when my husband died. I had proof to the contrary and believed this would be sorted out very quickly, I supplied copies of wage slips for the previous 2 years and had also claimed unemployment benefit which only a resident can do and again supplied this proof. A year later after having not much contact from my lawyer I met with him and asked why this hadn't been resolved, he then told me that the residency was not the issue, it was now to do with the valuation of my property. They believed it had been undervalued and this now took me above the tax threshold. No-one had come to my house to give a valuation they just said this was their opinion. At the time of my husbands death we had about €50,000 in the bank and our lawyer valued the house at €230,000, we took out a mortgage in 2005 for €37,000 (we purchased the property in 1991). Everything was in joint names. I was told that this mortgage is considered an asset not a debt as it was taken out after we bought the property, even though this money was not in our account anymore as we spent it on home improvements, and I have to pay it back when I sell the property. We had a mortgage when we bought in 1991 which finished 10 years later. I am now in the process of selling my property for €263,000 and I have to pay these death duties as soon as the sale goes through and would have to go through the courts to appeal which we all know would take years. Is there anything I can do to stop this money being taken as I believe I am being unfairly taxed and my lawyer is not very helpful.
Many thanks for any advice you can give.