Ridiculous rule of not sharing an arrangement letter with the client that you made an arrangement with

They have a rule that doesn’t make sense, I was paying them 500 monthly then I decided to make it 2000 so I can quickly settle the account. I made an arrangement with them to pay that 2k and when I asked them to share our arrangement letter they told me they can’t share it with me even though this arrangement is an agreement between them and myself however they are willing to share with companies that will require it from me.