The La Liga outfit are committed to purchasing the 24-year-old, who has spent the last two seasons on loan at the Anoeta.
In a complicated transfer, City paid £4.5m in the summer to his parent club Deportivo Maldonaldo
Rulli, however, was not keen on sitting on the bench at the Etihad Stadium and wanted to continue developing with Sociedad.
Under the terms of the agreement, City will bank a quick profit in January of £1.3m but also retain 30 per cent of his rights.
Spanish reports claim the Citizens also have a buy-back clause of £11.7m that can be activated in the summer of 2018.
If Pep Guardiola decides not to take up the club's option, Rulli's clause for other clubs is set at £29m, meaning the Manchester giants would stand to gain a further £8.7m - raking in a total profit of £10m - if anyone comes calling in the future for the player.