MADRID (Reuters) -Spanish insurer Mapfre said on Wednesday it had sold its stake in Bankia's insurance business to Caixabank 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.