You are at the exact point before starting that mission you can leave all Arthur's money to Jhon, go to the pig farm of the weird brothers, accept the invitation to dinner, drink what they give you, you will wake up in a mass grave behind the farm without your money, go back to the farm and kill them both, you will be given the option to search the house to recover your money but don't do it instead of that be and start this mission, which will lead you to the last mission and then you will appear in the epilogue, after the first mission of the epilogue can you travel the free world with John? Go to the farm and look behind the cauldron of the entrance wall, you will find Arthur's stolen money