Customer cannot remember transaction number but he knew the memo.
He wants to search transaction with memo as keyword via Global search
This is still an enhancement (Enhancement: 82717) - Global Search > Have the Memo Field (on Transactions) be a search-able field when using the Global Search
As a workaround:
1. Create custom field instead of using the standard memo field
a. Navigate to Customization > Lists, Records, & Fields > Transaction body Fields > New
b. Mark the Global Search box
2. On Applies To tab> Select the transaction where you want to apply the field.
3. Under Display tab> Select Main on Subtab.