Whom is a player bankrupt to if a Chance card tells him to pay $50 to each player and he can't pay?
There are indeed no instructions in the official rules on how to resolve multiple payments. A common mechanism in game design to handle such cases is to pay your debts in turn order. You go bankrupt to the player who you cannot pay. 

Example: turn order is Alice, Bob, Charlie, Daniel and Eve. It's Alice's turn and she is instructed to pay $50 to each player. Her entire liquid assets are $149. She pays $50 to Bob and Charlie, respectively, but since she's $1 short of paying the debt to Daniel, she goes immediately bankrupt and turns all her assets to Daniel. Eve gets nothing.

Turn order is clockwise, as indicated by the rules:


  THE PLAY... [...] After you have completed your play, the turn passes to the left.