From Old English 'dǣl' (part, portion), from Proto-Germanic *dailiz — dealing was originally dividing things into shares.
An agreement entered into by two or more parties; a large amount.
From Old English 'dǣl' meaning 'part, portion, share,' from Proto-Germanic *dailiz (portion, share), from PIE *dail- (to divide). The business sense grew from dividing goods between parties. Key roots: *dail- (PIE: "to divide, share