Greenback (as the name suggests) is a problem with the tomato fruit where the shoulders of the fruit fail to ripen properly leaving you with hard, tasteless parts of the fruit. Its causes are similar to whitewall, too much light and/or heat as the fruit are maturing or lack of potassium or feed whilst the fruit are growing.
Once the fruit are affected, there’s no way of improving the situation. The rest of the fruit will be perfectly acceptable but the green shoulders will never fully ripen and its best to cut them off and throw them away.
Solutions are pretty obvious: Reduce the sunlight on the tomatoes by shading the greenhouse; reduce the temperature in the greenhouse by increasing the ventilation and damping down paths; make sure you feed the plants adequately, choosing a tomato fertiliser with a decent potassium percentage; grow varieties which are less susceptible.