user: Question: Which player from GER has a transfer window of Summer, and a transfer fee of n/a?
Columns: ['Nat.', 'Name', 'Type', 'Transfer window', 'Transfer fee']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['GER', 'Oliver Kahn', 'Career end', 'Summer', 'n/a']
SQL:

assistant: SELECT "Name" FROM "Table" WHERE "Transfer window" = 'Summer' AND "Nat." = 'GER' AND "Transfer fee" = 'n/a';