In my business dealings over the years, this kind of scenario arose many times. It's simply a matter of stroking the client's ego (to start with, anyway). I had a client who I used to take to lap dancing bars to 'celebrate' a good deal. We could drop 2 or 3 grand easily in a night. It got to the stage when negotiations got tough, I would remind him that the chance of another 'evening out' was not to be missed. It worked every time.
All this and I was selling ..... Johnnie Walker Blue Label! (other whiskies are available). Mind you, he was a £10m a year customer.
I moved into a new job and my replacement was a VERY attractive girl, I mean really hot. You know, £300 an hour hot. I got a call from the customer 6 months after I left. He wanted me to come back as he missed our gentlemen's evenings. He said the new girl had nicer tits than me but he preferred my way of doing business!
Of course this goes on, regardless of quality of service/product. A pretty girl, a lap dancing bar, a hooker - all varying degrees of the same scenario.
To the OP, if you find the right SP, offer to pay her double her hourly rate (on the firm, of course), keep it as your little secret, then fuck her afterwards as a reward to yourself for being such a cunning bastard. How I miss the cut and THRUST of business!