Imagine a parent saying i will pay your rent but only if you live in a particular place? i will pay tuition but only in a certain school. i accept that this is not quite the same, but any parent who decides things for their grandchildren is overstepping her boundaries.
Oomis, and if the money was so that your child could live in Iran and/or go to Our Lady of Grace High, I think you would reconsider.
No one is responsible to support anyone. If someone decides out of the goodness of their heart, or in an attempt to direct their children/grandchildren on what they believe is the right path, to offer an incentive, the couple/parents are within rights to refuse such incentive. There is nothing wrong with it being offered.
In this case, the grandparent is allowed to buy shoes, but if the parents don’t like them, they can always resell them on E-bay.