Locally, It is my understanding that the brokerage firm owns all listings and is entitled to all agreed compensation in relation to the successful completion of a real estate transaction.
For clarifacation, where you state that the seller hired you, would appear to be legally inaccurate as the seller actually hired the brokerage firm, which you simply represented at that point in time.
Locally, brokerage firms may stipulate in their policy, that if a sales representative unilaterally decides to reduce the commission due on the successful completion of a transaction, that the reduction in question is taken from the representatives portion of the commission.
(In other words you cannot give away any part of the brokerage firms portion of the commission, only your own)
Locally there is generally policy that unless there is an express agreement to the contrary, it shall be assumed that two representatives working together in the same transaction will share the compensation earned equally on a 50/50 basis.
Further, that in the event that there is an express agreement between the representatives then all such agreements are subject to the approval of the principal broker of the brokerage firm.
These policies were developed to maintain harmony among all the representatives of the firm and avoid disruptive disputes over commission sharing.
If the accepted premiss is that the brokerage firm owns all the listings, then that would indicate that you are not in a position to offer to share anything that you do not legally own and are entitled to share.
Consider the position of an adjudicator of an arbitration hearing who must base their findings on the letter of the law and the law of equity.
The law of equity relates to being fair and impartial and which an adjudicator when weighing all the facts as may be involved in your particullar situation and depending upon the general practice of other brokerage firms in your jurisdiction, (legal precedents) and regarding the same issues may find in your favour.
Prior to taking any action, one might consider making a reasonable offer of settlement, in writing,of an equal split of the 80% commission and which might be found acceptable and still leave you with 40% of the earned commission and 20% more than what you have received to date.
Adjudicators have a tendency to believe that the party who rejects a reasonable offer of settlement are in fact being unreasonable and may render their decision accordingly.
Important Notice: This information is provided as basic educational information by the author and is not a substitute for the advice of an expert and/or the advice of a lawyer. There is NO representation as to legality, accuracy, correctness of the herein information and the reader is strongly urged to consult a lawyer in the relevant jurisdiction to ensure accuracy before acting on this information.