Interesting requirement but why they need to do this kind of control?
You have to ask them this question first. If only it is key sensitive fields, then you need special controls. if not just for segregation purpose, you have to try to say no to them.
SAP defines segregation by Finance and SD.
SD maintain general and sales area.
Finance maintain company code data or/+ General Data. These are controlled already through customizing in OB20.