There are two different issues involved: the cracked fuel line, and the Engine Core Upgrade (ECU). The fuel line fracture was caused by harmonic vibrations which only affected a “small” number of engines, and which takes about half an hour to fix.
Dunno who is paying for that, or what the total cost is.
However, the $2.4B ECU proposal is for upgrading the engines to “Block 4” capabilities - an improvement program, not a defect fix.