Shakhtar bought Solomon for $ 6m , they didn't develop him,
He had 18 months left on his contract when he joined Fulham, which is when Fifa declared that any players contracted to a russian or ukrainan club could suspend their contracts until June 2024.
FIFA extends and adapts temporary employment rules to address issues relating to war in Ukraine
Solomon had 6 months left on his (suspended) contract when he signed for Spurs and that contract would have terminated on 31 December 2023, before the 30 June when under the fifa rules the contract suspension was due to end. So when Spurs signed him he was in effect a free agent due to fifa rules - so Shakhtar's beef is with fifa not Spurs.
Even disregarding the fifa rules he had 6 months to run on his contract and with his injury record was not worth much - certainly not the 6m Shakhtar claim.
Spurs have played Shakhtar in a preseason friendly giving them the proceeds.
Given fifa's rules were ratified by CAS I doubt if they have a legal case against anyone at all.
I doubt if Spurs are that interested in retaining Solomon due to his injury record, and I doubt if anyone else will pay much, if anything, for Solomon in June.