Controlling data entry in a protected workbook

I have an excel workbook that contains a pricing calculator for the products we make. The worksheets are protected, allowing users to only put data into the unlocked cells. The workbook itself is protected to prevent users (inadvertently, we hope) altering the pricing tables.

However there are a few cells containing options that can only be made with a manager's approval. Therefore the user needs to be asked for a password when then try to change the value in these specific cells (discount, for example). If they cant supply the password (which the manager has), then the value should revert to what it previously was.

I don't really want to un-protect the workbook and/or sheet just to change these cells, as that will leave it very vulnerable

I guess this is possible a use of VBA - any ideas?