cross someone's palm with silver
Pay for a service; pay in advance. For example, I'll give you all the details, but first cross my palm with silver. This phrase alludes to the old practice of placing silver coins across a gypsy fortuneteller's hand before having one's fortune told. Today it is sometimes used in a jocular way to ask for a bribe or a tip, as in the example. [c. 1700] Also see grease someone's palm.