In a brief statement, it said that an agreement would be concluded with Fortis Group giving shareholders the right to any income and capital gain that exceed the financial charges of the state's investment.
A "reasonable" risk premium also would be factored into the Belgian state's holding, reported Worldbulletin.
BNP Paribas agreed late on Sunday to buy Fortis assets in Belgium and Luxembourg for 14.5 billion euros ($20.1 billion) in a share swap with the Belgian and Luxembourg governments.
This followed the Netherlands' move on Friday to nationalise the Dutch assets of Fortis for 16.8 billion euros.
Belgium will retain a 25 percent blocking stake in Fortis Bank Belgium and also will get an 11.6 percent interest in BNP Paribas.
The deals mean Fortis Group is left only with its international insurance business and a 66 percent stake in a 10.4 billion euro portfolio of structured credit products.
Analysts said Fortis shares, which were suspended on Monday, were worth only 3.5 to 4 euros. They closed at 5.422 euros in Brussels on Friday.