in AE we use "a marker" as a commonly understood equivalent of "an informal I.O.U."?
Remade in 1949 (retitled Sorrowful Jones), starring Bob Hope and Lucille Ball, and in 1980, starring Walter Matthau and Julie Andrews.... there was even an old Shirley Temple movie called "Little Miss Marker" where a father leaves his child as a marker for his gambling debt, promising to come back for her when he has the money.