No battery replacement granted

According to Apple, the battery is a consumable item.

iPhone Battery and Performance - Apple Support

"All rechargeable batteries are consumables and have a limited lifespan — eventually their capacity and performance decline such that they need to be replaced."

They also say that iOS 11.3 and later display text about Peak Performance Capability. I don't know what the text on your phone says, but if it says "Your battery is currently supporting normal peak performance," the reseller may be trying to save you some money, on a replacement that they think is not needed.

I want to pay for a new battery and they refuse.

Did you make it clear to the reseller up front that you wanted to pay for a new battery – that you were not asking for a free replacement? Or might they have gotten the impression that you were trying to get a free battery?

This is a user-to-user public forum.

This platform serves as a space for users to engage in meaningful conversations, share information, and exchange ideas related to Apple products. It's a community-driven initiative where users support each other by sharing their experiences and technical expertise in using Apple devices. While Apple Inc. is not actively participating in these discussions, the forum is a valuable resource for seeking guidance and assistance from fellow users who possess a wealth of experience with Apple products. Feel free to explore the discussions, ask questions, and benefit from the collective knowledge of this community.

Hence, it may not be feasible to help with any of the following:

1. Questions directed to Apple Inc - refer to the three links below
   1. Product Feedback - Apple
   2. Feedback - Weather - Apple
   3. Maps Report an Issue & Privacy - Apple Support & Report a problem with Maps on Mac - Apple Support
2. Hardware repairs & tickets - Apple Authorized Service Provider
3. Service grievances & questions --> Apple Repair and Repair Status Check - Official Apple Support
4. Warranty & Apple Care+ issues --> Check Your Service and Support Coverage - Apple Support
5. Order & Transaction related information -->
   1. Order Status - Shopping Help
   2. Viewing & Changing Orders - Shopping Help
   3. Genius Bar Reservation and Apple Support Options - Apple
6. Suggestions and Feedback for improvements --> Apple Inc says; "We read all feedback carefully, but we are unable to respond to each submission individually."
   1. Here is how for iPhone · Country or region* Select your country or region. · Feedback Type* Select feedback type. · Comments* Enter a comment.
   2. Feedback - iPhone - Apple
7. Shipping & Transportation queries - Shipping & Pickup - Shopping Help - Apple
8. Repair costs or product or prices of part service & replacement -
   1. iPhone Repair and Service - Apple Support (IN)
   2. iPhone Screen Repair & Replacement
   3. iPhone Battery Repair & Replacement
9. Product & Launch Detail etc.. --> Apple
10. Third-party apps and their behavior - How to contact an app developer - Apple Support
11. Any other issue --> Contact – Official Apple Support (IN) & Contact Apple for support and service