When voluntary sales of a part of any tract so assessed shall be made, it shall be the duty of the vendor to file a petition in the county court of the county in which the cause was instituted, establishing the drainage district against the purchaser, setting out the facts, and the court shall determine what part of the assessment shall be borne by the tract so conveyed and what part by the tract or parcel retained by the vendor, which decree shall be entered on the drainage record book, and certified by the clerk of the county, to the trustee, whose duty it is to collect the assessments, and the trustee shall make the change in the drainage assessment book and thereafter collect the assessments accordingly, and the cost of filing the petition and conducting the proceeding shall be borne equally by the vendor and vendee of such part of land.