It is very possible that FE requested the refund as good will for an imcomplete kit. It was easier than researching the feasability of using the heavier spring or convincing their customer (you) that it might be an acceptable alternative.

Years ago, there was a dealer installed Nivomat option for the 740. Standard springs with Nivomats bolted in place of regular tube shocks.

I absoulutely agree that the Nivos are a good auto level shock with a much better ride than air-lift types. All our cars came with them and I wouldn't convert.