The Spain international was sold to Sporting this month after spending two seasons on-loan in Lisbon.
Sold for €8.5m, Porro can be re-signed by Manchester City for a set price.
According to transfer expert Fabrizio Romano, City will be allowed to discuss a contract with Porro if they offer Sporting €20m, which is less than half the buyout clause (€45m) for other clubs.
City will keep tabs on Porro's progress as they seek a long-term successor for Kyle Walker, 32.
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