pin money

Small sum for incidental expenses; a small cash advance with a major contract might be called pin money.

Small amounts of money for incidental expenses, as in Grandma usually gives the children some pin money whenever she visits. This expression originally signified money given by a husband to his wife for small personal expenditures such as pins, which were very costly items in centuries past. A will recorded at York in 1542 listed a bequest: "I give my said daughter Margarett my lease of the parsonage . . . to buy her pins." [Early 1500s]

The noun has one meaning:

Meaning #1: cash for day-to-day spending on incidental expenses
  Synonyms: pocket money, spending money