Do not try to negotiate a lower price from the guy who just performed work for you. You reached an agreement and based upon your word, the vendor worked and completed his part of this verbal bargain. Do not try to beat down his price or negotiate a cheaper price at the last minute after he is finished and has completed his side of your mutual agreement. This is just one of many unethical actions to avoid.
Your temporary gain will cost you far more in the market. Even if the vendor caves in at this point after you have cheated the man and if asked, he most probably will not do business with you again. The worst part of this type of last minute arm-bending lies in the fact he will now go tell 25 people he knows you are not to be trusted. He will be warranted in his complaint about you and will do damage to your reputation that is ten times the amount of money that you unfairly extracted from him. Perform your duty and pay for work completed.
Clarify what you are buying in full clear detail, leaving no questions. This is one of the reasons why companies should issue clear, written, and specific purchase orders detailing the entire price, freight terms, advance payments, monthly deposits, progress payments, final payment terms if any, sales and use tax exemptions and everything else that is or is not included in the price.
Purchase orders need to be in writing. People have bad memories when money is involved. Get this great buy in writing. Negotiators change. People leave companies. Costs change. Situations become more difficult or market costs diminish the attractiveness of your agreement. Motivations of the parties ebb and it may be difficult to finish your business because of these changes. If all agreements were written originally, it will be easier to modify them if the contract needs change orders. Agreements can be changed and renegotiated but they are nearly impossible to iron out when everything important was only talked about. Your vendor will appreciate these written arrangements and regard you with utmost respect and will tell his friends the same.
Operate fairly and openly and you will have an abundance of market options. Do not be cheap and pull this type of last minute price deception or anything like this on your vendor just to make a few dollars at their expense. It will cost you many times more than the dollars saved in bad reputation out in the market place. Practice the integrity you tell people you value and cherish. Be honorable and everyone will want to work for you, a great idea.