Is the gift I received from the seller permissible, after I purchased expensive items for my senior manager, knowing that the cost of these items was not from my own money?

The gift given by the seller to the questioner is due to the purchase made for the manager. Therefore, the one who deserves it is the purchasing manager, not the agent who made the purchase, unless the principal willingly cedes it to his agent. This is supported by what was narrated from Imam Ahmad regarding the gift to an agent: if a man gives another a garment to sell, and he sells it, and then the buyer gifts him a handkerchief, then the handkerchief belongs to the owner of the garment. This is because the gift of the handkerchief was caused by the sale, making the handkerchief an addition to the price, and an addition made at the time of the contract becomes part of it.