Sonic Drive-In review: tipping

Do managers train servers to determine their own tips? Friday, August 2, 2019, I placed a happy hour order at the above location. I ordered a Route 44 Unsweet Tea. The total was $1.29. When the server arrived, I gave him a $20. He said he would be back with my change. He returned & gave me $18.00...$.71 less than I was owed. I told him that I was planning on tipping him but I wanted all my change. Also, my order was incorrect so I asked for it to be corrected. He returned with my tea but not my change. I reminded him & he said he would be back. I waited as I saw him serve several other customers. I realized I was not going to get my change without making a scene and did not have the time or energy so I left. HOWEVER... I will not return to that location. Also, this is not the first time this has happened & not just at this location. Please inform your managers to train servers that it is not up to them to determine their tips...that is up to the customer. I have never been to another restaurant where they tell me what I am going to tip unless it is auto gratuity for a large group. This practice is dishonest & unethical. If I experience this one more time, Sonic is off my list of destinations.