I need a document contract writer

Iam in a open probate. Iam executor of estate. My sister would like to be the purchaser of the house which is the probate inheritance for a lower price ($500,000) than the court referee appraisal. I would like a court standing contract written stating she will give me out of the lower cost split what would balance my total to what i would get if the house was sold at the court referee appraised price.