When people ***spar***[verb] (or have a ***spar***[noun]) over some topic, they have a friendly argument.
> Phelps invited Ted Frederickson, associate professor of journalism, to
> a ***spar*** on the issue of "homosexuals and the military".
>
> Phelps invited Ted Frederickson, associate professor of journalism, to
> ***spar*** over the issue of "homosexuals and the military".

ODO:

> **[spar][1]**  
> *NOUN*
> 
> **1** A period or bout of ***sparring***.
> 
> ‘Remembering the spars she had had with the brothers, she smiled.’
> 
> *VERB*
> 
> [NO OBJECT]  
> **1.1** **Argue** with someone **without marked hostility**:   
‘mother and daughter spar regularly over drink, drugs, and career’


  [1]: https://en.oxforddictionaries.com/definition/spar