Is it wrong of me to make my kids “owe” me money at their age?

Let me preface with saying that my kids have a decent sized piggy bank and money goes into it weekly. They’ve never used their before and never had any reason to. Let’s fast forward.

I took my kids to Target because I had to get some stuff and while there they asked for toys. Now, it had been a while since I bought them toys so I said ok just as long as each of their toys was under $20 (two kids would mean spending an extra $40). My son (4) quickly picked up some Pokémon belt with two Pokéballs and my daughter (6) picked out an LOL Biggie Pet. My daughter’s toy was $25 whereas my son stayed within budget. She was sad when she saw the price and kept looking for something else but nothing caught her attention. So I told her that it was fine, if she wanted it, she could have it, BUT when we got home she’s have to take $5 out of her piggy bank because she owed me.

We got home and she went to the neighbor’s house to open the new toy and forgot about it. She comes back in showing it to me and I simply tell her “Hey, don’t forget you owe me $5.” Without hesitating she runs to her room, grabs the $5 and placed it on my bed. I don’t know why I’m feeling so guilty about this though. I’ll probably end up putting it back in (without telling them) anyway. On one hand I feel like I should be teaching them being responsible with their money (even though they’re still small) but on the other hand I feel like they’re my kids so buying toys should be my responsibility (I’m their parents so duh it should fall on me) no matter the cost. I don’t know… let me know what you guys think or let me know if you’ve ever been in a similar situation.