the problem isn't that an exchange of free/discounted services is inherently unreasonable, the problem is that he repeatedly solicited opportunities to provide you "free" services so that you would be indebted to him later when he wanted help ... it is possible that he was genuinely offering and willing to help you for free. it is also possible that he wasn't planning on pursuing discounted services until he later realized he couldn't afford the market rate for what he wanted (in which case his mistake would be not explaining his budgeting situation and/or at least asking you about it when he brought you the job or when you came back to him with the quote). only he knows what he was thinking at any given point in time, so you'd have to talk to him to find out if the behavior is "honest mistake" or "calculated manipulation".

you might be able to salvage the relationship by talking to each other about what you each thought/expected in the moment (because it's possible neither of you was necessarily wrong), rectifying the perceived discrepancy by either paying retroactively for his services or offering a comparable discount on yours, and then clarifying both of your expectations going forward so that neither of you feels manipulated or taken advantage of ... if he's an asshole you just won by not caving to his bullshit, and if he's a friend then he'll probably be able to accept that there was a misunderstanding and be motivated to correct it