Where is the original battery for my phone, and why was it replaced without my consent?

After taking two cell phones to Gadget Paradise for screen repair quotes, I returned to find that they had already opened and repaired the phones without my consent. Needing the phones back, I accepted the cost and took them.

However, after one of the phones malfunctioned just two days after I started using it, I discovered that the original battery had been replaced with a different one. Gadget Paradise is the only place that had opened the phone, which leads me to conclude that the original battery was replaced by them without my consent - ******?