Is the amount of money that the solicitor took from the seller, without his associate's knowledge, lawful or unlawful?

It is permissible to take a commission from the seller, the buyer, or both, if this has been agreed upon. There is no specific percentage set for the commission; rather, whatever has been agreed upon and mutually accepted is permissible. The agreement made between you and your colleague is binding upon both of you, based on the Almighty's saying: (O YOU WHO HAVE BELIEVED, FULFILL [YOUR] CONTRACTS). So, if the agreement was to split the commission taken only from the buyer, then he has no right to what you took from the seller. However, if the agreement was for a general partnership in the commission without restriction, then you are obligated to give him half of what you received from the seller, and he is obligated to give you half of what he received from the buyer.