Sovereign, supranational and agency borrowers have for years held the upper hand over their investment banks. The banks may be about to get even.

What has long tipped the scales against the banks is the one-sided way in which their SSA clients mitigate exposure to loss, say from movements in currencies and interest rates. Any client that issues a bond in one currency and might want to swap it into another, for example, hedges against adverse exchange rate movements with a currency swap contract. But when a corporate takes out such a contract, both it and the bank must post collateral when on the losing side of the swap.