Think of it another way. You aren't buying anything.
What you are doing is paying Bosch to repair your faulty MAF. But instead of your one going to them for repair and the car being off the road for 2-3 weeks what they do is give you one they have already repaired and you give them back the broken one. The £20 is a security deposit in case you don't give your one back. If you're taking your one with you you don't pay the £20 and you just swap MAFs.
As DPJ was hinting they don't sell new ones anymore, just remanufactured.
As to descreening, no idea, depends on if they notice or care.