Hi Budi,
You can try as described below..
1. Change the area in Rental space-1 (Created with area 1000 m2), maintain measurement as 600 m2 with validity.
For example the tenant A surrendered 400 m2 on 01.04.2016, the measurements must be as below
From 00.00.0000 to 31.03.2016 - 1000 m2
From 01.04.2016 to - 600 m2
2. Create another rental space-2 with object validity 01.04.2016 and maintain measurement 400 m2
3. Create Contract on Tenant B with object Rental space-2, don't add any condition types so that only rental space-2 will be occupied without any cash flow.
In case if you are using Object cash flow (distribution postings) and distribution parameter is 'Object measurement', then i think you have to adjust through CO entries (Object to Object i.e. Rental space-1 to Rental space-2).
Rgds,
Srini