A few Ways.
Tell them to give you a list of what's neede,then buy it yourself.
Establish credit with a business,hardware store,colmado,construction supplies,and tell them to go there.
Easiest,is give the bare minimum to buy the parts needed,and "Pay-as-you-go" for projects that last more than one day.
KNOW your employees,tradesmen,mechanics well,before you use them.
Learn to say,"No,No Puedo Ahora.' when they want to be payed for incomplete work.
KNOW WHERE THEY LIVE!
References from other EXPats is very good.
You don't need to be "Un Amigo" to people working for you.
I dropped off my van to have the radiator repared.I left my "cell" # to call when it was ready.They dis.Everything fine.
Next day a got a call from the guy,"My little child is in the hospital and might not live if I can't pay for her operation!"
I told him "No,No Puedo Ahora",but call me if she dies,I'll send some flowers.
He never called,so I guess she survived?????
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Oh,and always carry a lot of small bills.They NEVER have "change",but don't want YOU to pay 100 pesos Less!