Friendships when debt enters the equation

I (M 21) have been friends with this guy since high school. He was a year ahead of me.

Back when covid hit and schools were closed, I decided to buy a guitar to keep me busy and at least learn a skill. I purchased the guitar and other accessories (guitar bag, capo and Strummers)

Almost a year later, when schools reopened, I wasn't really using the guitar because I was in school and I was a candidate preparing for KCSE exams. So he(the friend), suggests I sell him the guitar. I didn't think much about it so I agreed and we talked about the price for the guitar plus all its accessories, and how he'll make the payments because he didn't have the full amount. He upheld his end of the deal and paid almost half the price, and the balance was ksh.4,600. This balance is what causes complications later because he had some financial problems- so he says.

Long story short, 3 years have passed and still no transaction has been made. We haven't spoken much since then.

So today I decided to text him, to know how he is, and I casually brought up the story and the money he owes me. He immediately calls me out for being forgetful, says that he paid the full amount years ago. I'm not dumb and neither is he, so he's definitely lying and he's trying to gaslight me. A few 'long' paragraphs were exchanged, but I let it go.

I know other friendships end because of much more than this but it's quite heartbreaking.