Combine vinegar and currants in a small non-reactive bowl and set aside until currants are plump (10-15 minutes).
Heat a large frying pan over medium heat, add half the oil, all the onion and all the garlic and stir occasionally until soft (7-10 minutes). Add tomato and stir occasionally until thick (5-7 minutes). Add sugar, pine nuts, currants and vinegar, stir, season to taste and set aside.
Heat remaining olive oil in a large frying pan over medium-high heat, add sausage and turn occasionally until golden and cooked through (7-10 minutes). Remove from heat, slice thickly and serve immediately with sauce.