Just wondering, all ethical aspects aside as other posters have brought them up and you've replied, why exactly you're anxious that you continue a friendship with this man?
If someone is my friend, they offer to help me if they're in a position to when I need help.
If they're not my friend, they make it clear upfront that they are doing a job of work for me, rather than a favour.
Examples: (1) there's a very nice chap who worked for my late husband and I doing 2 shifts each week for 16 years. He can be kind and thoughtful - during the week long power cut in our rural area last winter, he took me up to his home where I was able to shower and enjoy a hot meal with his wife and him. He still does the odd job for me but he would never, ever, consider not charging his full rate, plus fuel costs since I've moved further away.
(2) Our farrier, who over many years has become a real friend not only refused to ever put his rates up for us as he did his other clients, but when I moved and was upset at the prospect of bare wooden floors with just a few rugs down as I was unable to afford to carpet my house, gave me free and with no strings attached a roll of carpet big enough to do my whole house. He also moved my furniture in his horsebox to save me the cost of a removal firm.
Your friend seems to be in the example (1) category. So what are you getting from the friendship that makes you anxious about rocking the boat? If you know the answer to that, it'll help you know how to deal with him - or not - going forwards.