1. Call your head hunter. Any employer who thinks like this will lose its top producers soon enough and be stuck with the dregs soon enough. It will then circle the drain until it falls in. You want to be ahead of the pack.
2. This thing needs details and real answers in a written policy (if they're going to hold you to it). Per diems were put in place because they are a way of letting people use their own discretion and cutting down reimbursement and expense-processing costs. In other words, 2 day trip at $90/day means $180. No receipts, no nothing. You want to eat peanut butter crackers your brought from home and pocket the $180, that's up to you AND it's not fraud.
3. So, the details:
A. Under what circumstances are you able to go outside the contract (no restaurant close by, dietary needs, client entertainment, etc)?
B. Is it a lower per diem outside the contracts (in other words, what if you want to eat at a 5-star restaurant and pick up the difference yourself)?