How would you effectively separate Sales and Delivery, while ensuring that your joint goals are achieved?

No corners should be cut. The Sales team must agree on a deal that's achievable by the PM, good communication and constant feedback between Sales Team & the PM is of a paramount importance to enable both parties to reach the goal.

By reaching out to potential clients

While I in principle agree, the fact of the matter is that certain pressures are applied to meet both company and personal sales target which ultimately mean that corners where possible can and will be cut. Obvious ways to circumvent this is to instead offer the client certain incentives but without applying unnecessary pressures on the delivery channel. What can often be the case is that the company Sales targets do not align with their capacity to deliver.