MADRID, Dec 31 (Reuters) - Spanish construction group ACS said on Friday it will make a net gain of at least 2.9 billion euros ($3.28 billion) from the sale of its industrial unit to French rival Vinci SA (SGEF.PA). The two companies, which had reached an agreement on the 4.9 billion euro sale in March, signed the sale deed on Friday after securing all the regulatory approvals, ACS said in a statement. Both companies had said they expected to close the transaction by the end of 2021. The Spanish company expects to receive an additional 600 million euros over the next seven years tied to the development of the unit sold to Vinci over the period. ACS and Vinci agreed to create a joint venture firm 51% controlled by Vinci to operate all the renewable assets developed by the business unit.