The evaluation result for function call DOLLAR(1234.567;2) is just the raw numeric value itself. Without dollar sign, without thousands separators and without cutting/extending the value to the given number of decimal places.

In Method org.apache.poi.ss.formula.functions.new Var1or2ArgFunction() {...}.evaluate(int, int, ValueEval, ValueEval) for Function DOLLAR there is a comment:
// TODO - DOLLAR() function impl is NQR
// result should be StringEval, with leading '$' and thousands separators
// current junits are asserting incorrect behaviour

I think it's useful to document the difference from the expected behavior here in bugzilla, so that others can find it fast(er) when they search for unexpected DOLLAR-behavior.