We analyze dynamic capital allocation and risk sharing between a principal and many agents, who privately observe their output. Incentive compatibility requires that agents bear part of their idiosyncratic risk. The larger the agents' risk exposure, the larger the rents the principal can extract from them. The optimal mechanism can be implemented as the equilibrium of a market where agents exchange goods for money, needed to pay taxes. Inflation affects agents' portfolio choice between risky capital and safe money. To implement the optimal mechanism, the principal targets an inflation rate such that agents' risk exposure is the same in equilibrium and in the mechanism.