Tax is a difficult area to translate because every country has its own querks and there is thus no exact equivalent. This is a specifically Danish variant. It is an allowance or deduction which reduces the tax payable on earned income - it's purpose is to give equal conditions (hence
ligningsmæssigt) to wage earners in relation to the self-employed. An example is the "befordringsfradrag", given as an allowance against the cost of travelling to work where this is greater than 12km. Gyldendals Røde Fagbog gives "tax relief" but I think your own suggestion is good enough. Maybe add an adjective or two before it, like "supplementary tax allowance" or "work-related tax allowance".
http://www.tax.dk/lv/personskat/A_4.htm
The translation workplace 
