Martins was then farmed out on loan to the Principality side in January and has flourished enough to convince the powers that be to make an offer for 100 per cent of the player's rights, with talks currently underway.
Sporting's involvement may complicate any deal, however, with the Portuguese side still claiming that they are owed some amount of compensation after losing the player for free. Reports claim the club are looking for a figure in the region of €20m (£17m/$23m).
If an amicable agreement cannot be reached between the clubs, the matter will be taken to court and a judging panel would set the compensation fee.
For now, however, talks between Sporting president Frederico Varandas and Atletico's management are ongoing, with the latter eager to settle the issue and move on to sporting matters.
A situation that might work for both clubs is the Spanish side offering a player as compensation and it is understood that Sporting are not against the idea.
One possible name on the table is 25-year-old striker Luciano Vietto, who has made only 25 Liga appearances for the club since joining from Villarreal in 2015.
Another player who could be of interest to Sporting is Andre Moreira, a 23-year-old goalkeeper who is currently on loan at Feirense. Moreira is yet to make a single Liga appearance for Atleti and his contract runs out in the summer of this year.
With that in mind, a player exchange would see both clubs reach their goal: Atletico would no longer have to worry about legal issues, while Sporting would be strengthening their squad.