I think karma exists op, just maybe not in the way of £4 to £100. I know you're going to hand it in /try and trace the owner, hopefully you'll get it back just in time for Christmas if no one claims it.
Years ago I was out shopping with my parents. I was 15 at the time. Luckily I spotted the small purse on the floor before my dad did. Picked it up, there was around £300 in there plus a Yale key, pic of a child and folded up tiny copy of a child's birth certificate. I wanted to hand it in. My dad badgered me the whole way to a police station saying we should keep it, we could go away for a weekend, I could have some of it. He was getting more and more angry I wouldn't hand it over. He then tried wheedling, let me see it, there might be ID etc. I get we were poor, horrifically so, but I couldn't keep it. I walked into the police station and just started signing it over, as a woman flew in with toddler in her arms just sobbing her heart out. She asked if anyone had handed a purse in as the policeman pointed at me. They asked for her to identify it, but as it had a picture of the child she had in her arms, and she could recite all the info on the birth certificate they handed it to her. She threw her arms around me. She was a single parent. She'd saved for over two years to take her little boy on his first break away and the money was for that. She offered me £20 and I said no, but her little boy more ice creams. All the time my dad stood there saying how amazing I was and he was so proud of me, he'd brought me up right
(there's a reason I'm NC with the twat now).
Anyway, my karma is that the couple of times I've lost something it's always made it's way back. My exH lost a bracelet on NYE I'd given him at the start of our relationship, it had huge sentimental value. I posted on FB, within 3 days we had it back. Someone's dad found it (he didn't have FB) whilst walking the dog on NY day. He passed it to his sister, who gave it her daughter thinking her bloke might like it. The daughter saw my FB post and contacted me immediately. The relief of it coming back was awesome. We sent a huge bouquet of flowers as a thank you.