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.
Jurgen Klopp has no doubt Liverpool belong in the Champions League quarter-finals after confirming their progress with a goalless draw against Porto.
Real Madrid boss Zinedine Zidane believes their season is recovering as the defending champions cruised into the Champions League quarter-finals.
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Reading manager Jaap Stam defended his decision to give 17-year-old centre-back Tom Holmes his senior debut in the Sky Bet Championship draw against Bolton.
Alex Neil praised Sean Maguire after he turned match-winner Preston once again in their 2-1 victory over 10-man Bristol City at Deepdale.
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Ipswich Town manager Mick McCarthy felt his side were worthy winners after securing a 2-1 victory at Sheffield Wednesday.