You haven't provided enough information. Lets assume the stat is Strength and the character is a young (-4 Str) Halfling (-2 Str).
Young Creature: Ability Scores –4 Strength, –4 Con, +4 size bonus to Dex.
Remove the young template and the racial modifier from the current stat (4 + 4 + 2 = 10).
Adjust the base ability score up 3 (3 buy points to go from 10 to 13).
Re-add the young template and racial modifiers (13 - 4 - 2 = 7).
As racial adjustments (and presumably templates) are applied after generating ability scores it would cost 3 points to raise that stat to 13; then once you apply the -6 from race and young he'll have the 7 he wants.
See the second line in Generating Ability Scores in the PRD section for Ability scores.