Malaga will receive a payment of €1 million from Real Madrid if Los Blancos win La Liga this weekend.
The Costa del Sol club sold Isco to Real in 2013 for a fee of €27m and, according to reports in Spain, that deal included €3m in variables if the midfielder helped his new team win the Primera Division in his first five seasons - with a payment of €1m agreed for each title.
Madrid have claimed two Champions League crowns since Isco joined, but they have failed to win La Liga in that time, so none of those extras have yet been paid out.
However, Zinedine Zidane's side can clinch the title with a draw in their final game of the season on Sunday after beating Celta Vigo 4-1 on Wednesday - and that last match is away to Malaga.
The Andalusians, coached by Madrid legend Michel, have little to play for themselves and will finish between 11th and 13th in the table.
Malaga beat Barcelona 2-0 in early April and Michel said after that match: "Real Madrid will be the champions, but a Malaga man from Real Madrid has lent them a hand."