That is not even remotely close to being true!
Taking into account inflation, £26m per year (£500k per week) in 2020 is the equivalent of paying someone £7.5m per year (£144k per week) in 1980 when Keegan joined Saints.
The average wage for a top league player in 1980 was... £550 per week or £26k per year!
So I'm confident Southampton did not pay a player £144,000 per week in the 1980!