Contract theory: Incentive contracts (when agents have multiple tasks) models by G. Baker and Holmstrom & Milgrom
Holmström and Milgrom assume that the agent exhibits constant absolute risk aversion. This implies that if you change the total wage of the agent with a lump sum transfer (that does not depend on effort), you will not change the incentives. Hence, to find the optimal contract, you can find the contract that maximizes total surplus subject to incentive compatibility, and then subtract a lump sum amount to make the agent indifferent between accepting and not.