Miyagi is the worst friend anyone could ask for
The guy has an affair with his best friend's fiancee and then flees the country leaving them both dishonored. Then he refuses his so-called friend any chance of regaining said honor. He leaves the girl he supposedly loved much worse off than before. Her reputation is in tatters at this point.
Supposedly teaches Daniel karate so he can learn to solve his own problems, yet his dependence on him just grows to questionable levels. Later, he refuses to to train Daniel for the 85 All-Valley. Shouldn't that be Daniel's decision as to whether or not he fights? That was manipulation at its finest. Daniel would fight how, who, and where Miyagi told him to fight. Daniel also pretty much agreed to work in the bonsai shop for free. Do you know who else works for free? Slaves.
Let's delve deeper into Miyagi's ultimate plan for Daniel.
First he teaches him karate. Daniel is one of the few outside of the Miyagi family to learn their karate. Miyagi saw the makings of someone he could control. Miyagi is getting old and won't be able to fight forever. However, Daniel knows six moves at best.
The second thing he did was trick Daniel into giving him his college money for his bonsai store. Nobody seriously thought Daniel was college material did they? You may think this was Daniel's decision, it wasn't. Miyagi conditioned him into handing over the dough. More devious is that it's at Miyagi's behest that Daniel's name be on the lease too. The store is just a front.
Third, Miyagi deliberately sends Daniel over to the store where Jessica is. Notice his reaction to meeting Jessica Andrews is not the typical red-blooded male reaction. All he can think about is how Mr. Miyagi will react when he hears. Miyagi has planted the puppet strings on Daniel. It's now all about what Mr. Miyagi will think.
Miyagi-Daniel is more like a Don King-boxer relationship. Miyagi would borrow against the bonsai store's credit to set-up an underground fighting ring, put Daniel in a bunch of overhyped/rigged matches, and then keep all the money for it. After all "win lose no matter". It sure doesn't as long as the spread is covered!
Daniel never thinks for himself. Getting him to sign over power of attorney would be easy. Miyagi would flee town with all the money and stick Daniel with the bill to the bank for the debt on the store. All Daniel would be left with is debt.
Lowly janitors without friends do not take interest in young teens for wholesome reasons.