In terms of contract, I have found this works best (recommended by my lawyer (a former ASF lawyer)): 30% upfront, 50% at first delivery, then they have 2 months to find bugs, final delivery is when the bugs are fixed that's when you take the last 20%. This way, you're certain to always get 80%.
If they want the liberty to "change their mind on the fly" then they should buy man-days which is a completely other thing. Or, they can buy supplementary mandays for whatever they want at the daily rate without changing the contract, or, they can just wait to finish this contract and start another one.
If they want the liberty to "change their mind on the fly" then they should buy man-days which is a completely other thing. Or, they can buy supplementary mandays for whatever they want at the daily rate without changing the contract, or, they can just wait to finish this contract and start another one.