As the mandator, you are required to cooperate with your mandatary and you are obligated to:
advance any sums required for them to execute the power of attorney;
reimburse them for all reasonable expenses they incur fulfilling their duties; and
pay them the compensation they are entitled to.
Let’s say your mandatary is supposed to sell your car and the transmission fails. Your mandatary pays for the repair but isn’t able to close the sale after all. You will still have to cover their expenses, assuming they weren’t at fault.
You will also have to pay interest on those expenses, which will be calculated from the day they were incurred.