Heat the oil in a heavy based pan over a medium heat. Add the shallots, garlic and a pinch of salt and sweat down without colour until the shallots soften. Pour in the Port and 20ml balsamic vinegar. Cook over a medium heat and reduce by three quarters. Pass the reduction through a fine sieve. In a clean saucepan bring the veal jus to the boil and add the reduction. Season to taste with salt and freshly ground pepper before passing through a sieve lined with a double-layer of muslin cloth into a clean saucepan. Add the remaining balsamic vinegar to taste just before serving and finish by whisking in the chilled butter piece-by-piece until fully incorporated into the balsamic jus. Adjust the seasoning if necessary.