Is it permissible to take from the father's money, gifted to a son, as compensation for a loss, without the other siblings, and does the prohibition cease with their permission, and is that considered an injustice in giving, and must it be returned later?

There is no harm in singling out some children for a gift due to a legitimate reason, such as need, illness, large family size, preoccupation with seeking knowledge, or withholding the gift from some children due to their immorality, innovation (bid'ah), or using money for sinful acts. If there is no such reason but the siblings consent, then the harm is also removed.