Save yourself the $5k.
You could fix this easily:
1)make sure you have a valid trust drawn up before you do anything further;
2) Write up a new contract (Offer and acceptance) on the same terms as the old one (and get everyone to sign and the agent to confirm in writing that he only wants one commission!)
3) Lodge both contracts at the OSR with a completed cancellation contract (
http://www.dtf.wa.gov.au/cms/upload..._For_Exemption_For_Cancelled_Transactions.pdf
and an explanation at item 1 of the form that the transaction has not been carried into effect because it was entered into in the mistaken belief that you were acting as trustee of the XYZ trust when in fact that trust was not yet in existence.
You could formalise all this (if you wanted to) by getting a lawyer to draw up a Deed of Agreement (between you and the seller and the trustee of the trust) but of course that would cost $$$$.
So long as the OSR accepts that they are not being dudded (that is that really there is a "sub sale transaction" going on (See fact sheet on "Cancelled Transactions -http://www.dtf.wa.gov.au/cms/uploadedFiles/_State_Revenue/Duties/Cancelled_Transactions.pdf?n=442)
you shouldn't have any problems. If you do, you'd be up for stamp duties on both contracts (which you have said you are prepared to pay anyway!).
On the other hand, if this works (and it will) you will have saved $5k so you should feel free to give 10% of that to a charity of your choice...
Any problems, feel free to write me an abusive email
)