Well 2 ways:
1. You buy scrolls from the shop, each scroll triggers an event, the event triggers a menu of "which character should get this skill?" (Assuming you have more than 1) and the event also reduce the amount of scrolls in your inventory by 1, hence erasing the scroll
OR
2. Parallel event in the shop map, checking every 1-2 seconds for the existence of said scroll in your inventory, upon finding one it deletes it and grants the skill to whoever you think should get it by default or pops a menu of options.
Option 1 is the better one.