At what stage do you pay your mortgage broker?

Were in the process of getting a mortgage. Our broker is now asking us to pay him. We have had our mortgage offer, i think alls we need to do now is instruct the solicitor to do their searches and exchange contracts. Are we supposed to pay now or when the deal is done and we have our moving in date? The reason i ask is because he has been ripping us off throughout the whole process so he can get commission but we are first time buyers so we have made it very easy for him to do so. Thank you in advance:)

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