Contract negotiations depend on the company and the sector in which it operates. Apart from fixed payment jobs, pay can be taken to be negotiable. When you start to negotiate, remember that in Germany pay is expressed in hourly or monthly rates. Questions of pay are negotiated by the executive responsible (zustandige Fuhrungskraft).
Besides your pay, you can negotiate extra benefits, such as holiday pay and annual bonuses, provided they are not already included in the company's contract.