BRASILIA, April 28 (Reuters) - Brazil’s federal public debt rose 0.85% in March from the month before to a fresh record high 5.24 trillion reais ($972 billion), the Treasury said on Wednesday, adding the total domestic debt stock rose 0.7% to 4.99 trillion reais. The Treasury also said its liquidity cushion, essentially an emergency cash buffer, surged 20% in nominal terms to a new high of 1.12 trillion reais, while the average cost of servicing the public debt stock fell to a new low of 7.64%.