One of our investors is requiring us to provide a breakdown of the lenders credit so that they can recalculate the APR and test for high cost restrictions.

Can anyone give me a cite that says this is necessary?

An investor can require anything that they want to. If they want you to breakdown the credit to sell the loan you are stuck with that requirement. Now, if they want you to do that in the actual consumer disclosure documents, that is a whole other issue.

But they're saying they need it to determine what the finance charges are - why should it matter? It's just a lender credit.

If you showed a finance charge on the HUD-1 but specifically credited it back in the lender credit it would not be a finance charge. Maybe, you would have to ask them.