Three weeks ago, you listed a luxury property which has been heavily marketed via professional photography including video, several websites, social media and printed media such as papers and magazines. The Office Manager has informed you that the vendor hasn't paid any of their contribution and asks you follow this up as a matter of priority.

You go to the file and discover your oversight - you did not have the vendors sign off on the Marketing Plan, nor was it attached to the listing agreement, although you have written 'see attached marketing plan' at both the 'reimbursement of fees - when and how' and the 'property to be advertised / marketed' section.

You have just now called the vendors asking for payment and they refuse, stating that they didn't sign anything which makes them responsible for payment and you have mislead them into thinking the commission was inclusive of marketing.

What do you do and why, quoting any relevant legislation?