It will not come under IT Declarations. It will be under Claims only.
For your requirement, leave that amount field emty i.e don't show defalut amount value.
Let employee fill the amount as per supported documents.
During submission, you can validate this requested amount with 1.5 times of basic pay and allow to create the request. For validation, implement the BADI HRPBSIN_VALIDATE_REQUEST.
At approver level he needs to check the documents and approve it. If the amount needs to be modified then approver can modify and approve it. You can allow approver to modify the request.