CGT on Spanish Sales - Not Accepting Invoices

Just been reading through your fantastic forum - lots of great advice. Hoping you can help me....We have recently sold our Spanish house for 650.000 Euros. We have had the 3% retained from the purchase price and have employed an accountant to "fight for this back". We bought the land in 2005 and built the house and paid our builder, architect etc officially and have all the correct invoices which total this amount. In theory this means we do not owe any CGT. HOWEVER our accountant says that on our "declaracion de obra nueva" signed at the notary in 2005 our architect put an estimated cost of 200,000 Euros for the build of the house and the hacienda will only use this to base their calculations on and not our invoices. They will calculate the CGT as 650.000 Euros - 200.000 Euros. Bearing in mind we have legitimate invoices for the full amount I can not believe we would be eligible for paying around 95000 Euros in CGT. Any advice you could give me would be really appreciated. Thank you