MADRID, Dec 29 (Reuters) - Spanish insurer Mapfre (MAP.MC) said on Wednesday it had sold its stake in Bankia"s insurance business to Caixabank (CABK.MC) in a 571 million euro ($645.12 million) deal, although it has begun arbitration that could increase the value. Caixabank, which took over Bankia in March, has agreed to pay 324 million euros for Mapfre"s 51% stake in Bankia Vida life insurer, adding to the 49% it owned. It will also pay 247 million euros to end a contract to distribute non-life insurance, both companies said in separate filings to the Madrid stock market regulator. The combined amount represents 110% of the value of the two businesses as evaluated by an independent adviser, but Mapfre said it opened an arbitration process to demand Caixabank pays 120% of the value. "The parts will submit to a process of arbitration an additional compensation for up to 52 million euros," Mapfre said in a statement. Mapfre said the deal would yield a 171-million-euro profit that it will use to improve efficiency in Spain and Italy. Caixabank expected no significant impact on its accounts in 2021 from the transaction, adding that the 49% stake in the life insurance business contributed 29 million euros in the nine months of 2021 it was under Caixabank"s control. It said its 4.3 billion acquisition of Bankia was aimed at boosting profitability and reducing costs. ($1 = 0.8851 euros)