The toughest calculation in the sales contract deals with taxes. Real estate taxes are billed in one of two ways, either they are paid in advance or in arrears.
In locations where taxes are paid in advance, the seller has already paid a portion of the tax for the time you will be owner of the property. Therefore you will need to credit the seller for that amount. In cases where taxes are paid in arrears, the buyer will end up paying taxes for the time that the seller lived in the house, so the buyer will get the credit.