Fernando Tatís Jr. was 18 years old, just a low-level prospect from the Dominican Republic trying to work his way up in the San Diego Padres farm system, when he made a financial deal that would impact his entire baseball career. And it wasn’t with the Padres. Tatís signed a contract with Big League Advance, an unusual investment fund that pays minor-league players money up front in exchange for a share of their future MLB earnings.