Bad service, nissan refuse to give me my car

took my car to @nissanza for 105k service... thy went to do 90k servce. I told them i hv servce plan frm 105 since my standard sp hs expired. Thy refuse to gv me my car sayin i hav to pay them R4k. I need my car pls im runnin a taxi busines and its been 2days now. Its not my fault that they did 90k service wer as i yold them to do 105.