It's been arranged that I'll discuss the matter with him tomorrow.
Literal
Tomorrow him [with-と] that matter [object-を] discuss [arranged-ことになっている].
~ことになっている expresses a standing arrangement — something decided by external forces or mutual agreement. 話し合う is a compound verb (話す + 合う) meaning 'to discuss mutually' — the ~合う suffix adds reciprocity.