Is it permissible for unmarried children, during their father's lifetime, to demand compensation from their siblings who sold their properties at higher prices, for a marriage gift that their father alluded to but did not formalize in writing?

Marrying off children is part of the father's obligatory maintenance if he is able, and not a gift that requires justice among children. So, whoever needs to marry and is unable to do so, it is incumbent upon the father to marry them off. If the father dies after marrying off some of his children, the remaining children have no share in his estate other than their legal inheritance. It is not permissible for the father to bequeath money for the marriage of younger children as compensation for what he spent on the older ones. Nor are the siblings who were married off obligated to return what the father spent on them from their inheritance.