What is the ruling on selling a pharmacy decoration owned by a friend for a price higher than requested without his knowledge, and is the commission I took for mediating a car repair permissible?

If the decor is sold before it is owned, the sale is not permissible, as the Prophet (peace be upon him) forbade selling what one does not own. However, if there was a promise to sell, and then it was sold after acquisition of ownership, the sale is permissible.

Regarding bringing the mechanic, if no wage was agreed upon and you are not known for undertaking this work for a fee, then it is not permissible to take an extra amount in addition to the mechanic's wage, and it must be returned or the client must be asked for their absolution. However, if you are known for undertaking this work for a fee, then you are entitled to a customary wage.