if it works, then you are using it at the lower speed.
SO again, you have options:
1. Keep it as-is.
2. Return the faster memory for matched memory - you are back at the first risk issue.
3. Replace the slower memory with faster memory - ditto.
If I were in your shoes I'd leave it alone at this point in time. You're not harming anything and there are risks involved with any alternate choice that you already overcame with your current option.
Thanks I would keep it as-is. As I am not harming anything, faster one will work at lower speed so no problem. Thanks