Is it a line or is it the phone?

My sister-in-law has a good deal on cellular phones and phone line packages from M1. B1 have been wanting a new phone for long time. My wife has promised to get him one provided he fulfil certain obligations. Meanwhile, I am reading this book "Why we do what we do" on understanding Self-Motivation and just learned that giving rewards for doing things is less motivating than doing it for intrinsic reasons. Hence my position was to get rid of the strings. Unfortunately, when it come to implementation, I told B1 to choose a "free" phone that come with a contract. He was very disappointed and angry. I explained to him rationale and logic. I was worried to give him something that will make him keep up with the Jones(Lims in Singapore context). This may start a value or behaviour to keep expecting and demanding more and more material things to satisfy his desires. He has a complete shutdown. He does not want a phone anymore. He thought we have no intention of giving him the phone and just keep lying to keep him happy for a while.

We have a threesome meeting i.e. B1, Mummy and Daddy and have the issue clarified. I was wrong in my assumptions and unfortunately (for me) Mummy is right again. He only wanted the phone. The line was secondary. So we agreed to get him the phone. He just have to fulfil some conditions. He is still not very happy with this whole episode and I am very uncomfortable. Home life can be challenging and complicated...