It's better to have a lesser but certain advantage than the possibility of a greater one that may come to nothing.
This proverb refers back to medieval falconry where a bird in the hand (the falcon) was a valuable asset and certainly worth more than two in the bush (the prey).
A golden key can open any door
Money opens any door.
This notion must be as old as money itself. The first person who is known to have written it down is the English playwright John Lyly, in Euphues and his England, 1580:
Who is so ignorant that knoweth not, gold be a key for euery locke, chieflye with his Ladye.