The maximum period of return visa is 3 months.
The maximum period of residency proof (also a stamp on the passport) is 6 months.
Return visa can be used for travelling outside of the country.
The stamp of residency proof is only a proof of your legal residency in Holland. Usually it is used for applying a work permit.
In your case, you need return visa for 6 months. You'd better show the document to the IND staff and request for a longer return visa. It depends on luck. But there must be a way out. Don't worry. Go to IND and explain your situation.