I find the whole thing a bit odd. If princess Margaret was that strapped for cash that she decided to sell jewels she inherited from her grandmother would it not be more profitable to sell them with their royal pedigree? We've seen pieces with royal connections get much higher prices than equally beautiful pieces without a royal link.
From a financial point of view it does not make sense. Besides the Queen was wealthy enough to give her sister a similar amount in exchange for the jewels as they now got in the sale. That is assuming Elizabeth II would have agreed to such a thing. I think she would have simply given her sister the money she needed.