Selling financed car

Hello dear friends of the MOTOR-TALK, I have a question here. Hope you have a solution. I and my girlfriend and I financed a car 3 1/2 years ago. A Citroën ds3 Racing. Now we got offspring and the car has become too small for us with 2 children. The car has a residual debt of about 750€ at the bank. According to several dealers etc. it turned out that car is only about 5500-60€ worth. Since you can not give the car in payment because a overfinancing is not allowed, we are now facing the problem that we don’t know how to get rid of the car without taking up 2 financings. Does any of you have any idea what else to do?