Is a father giving his car to his son, to the exclusion of his daughter, considered a form of injustice in giving among children, especially if the father covered the expenses of the daughter's marriage?

The Shari'ah commands justice among children and prohibits preferring some over others. The preponderant opinion is that it is obligatory to be just among them in gifts, and it is not permissible to show preference without a valid, recognized need.

Therefore, it is not permissible for you to give the car to the son. If you do, you must either revoke the gift or give the equivalent value to the daughter so that no favoritism or preference occurs for either of them over the other.

If you were to give your daughter money equal to the value of the car, there would be no harm in that, because justice would have been achieved.

If you will not give the daughter the equivalent of the car, then it is not permissible for you to gift the car to the son. However, if he is in need of it, you may gift him its usufruct and allow him to use it until his need ceases, and the car remains your property, registered in your name.