I think I get it, you're saying that if you find the costs of rehab to be more than the bank's appraised value, you want to walk on the deal....right?
If this is on an REO offer, I bet it's not accepted if you put such a contingency in the contract. Basically, what you are saying is that "hey, I think I want to buy this property to flip, but I don't know what it will cost me nor will I know what it's worth until I see your appraisal" When they see that you can't do your due dilligence, they may not want to look closely at your offer...just an observation from experience. The fact that you are an investor and that you are financing your deal already puts you behind the 8 ball so to speak with an REO, IMO.
Another aspect is that you want the bank to basically loan off your CMA rather than the apprisal they order from their approved appriaser list and through the hoops that must jump through to obtain an "independent" appraisal.....won't happen in your life time!
No one will get a lender to loan off an appraisal or assement obtained and paid for by the borrower.
If you need an out, make your agreement with your lender and make your offer subject to financing. If you are telling your lender that you need X dollars to make the deal work, and apply for that amount and it does not appraise out to support that loan amount, you won't get the loan. Good luck!
I suggest you belly up and do the leg work and if you can't get access, then reflect that in your offer. Bnaks don't assume buyer's risks.